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jdk/src/hotspot/share/runtime/synchronizer.cpp
David Holmes c202bd705e 8250606: Remove unnecessary assertions in ObjectSynchronizer FastHashcode and inflate 
Reviewed-by: dcubed, coleenp
2020-08-06 21:03:18 -04:00

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/*
* Copyright (c) 1998, 2020, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#include "precompiled.hpp"
#include "classfile/vmSymbols.hpp"
#include "logging/log.hpp"
#include "logging/logStream.hpp"
#include "jfr/jfrEvents.hpp"
#include "memory/allocation.inline.hpp"
#include "memory/metaspaceShared.hpp"
#include "memory/padded.hpp"
#include "memory/resourceArea.hpp"
#include "memory/universe.hpp"
#include "oops/markWord.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/atomic.hpp"
#include "runtime/biasedLocking.hpp"
#include "runtime/handles.inline.hpp"
#include "runtime/handshake.hpp"
#include "runtime/interfaceSupport.inline.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/objectMonitor.hpp"
#include "runtime/objectMonitor.inline.hpp"
#include "runtime/osThread.hpp"
#include "runtime/safepointMechanism.inline.hpp"
#include "runtime/safepointVerifiers.hpp"
#include "runtime/sharedRuntime.hpp"
#include "runtime/stubRoutines.hpp"
#include "runtime/synchronizer.hpp"
#include "runtime/thread.inline.hpp"
#include "runtime/timer.hpp"
#include "runtime/vframe.hpp"
#include "runtime/vmThread.hpp"
#include "utilities/align.hpp"
#include "utilities/dtrace.hpp"
#include "utilities/events.hpp"
#include "utilities/preserveException.hpp"
// The "core" versions of monitor enter and exit reside in this file.
// The interpreter and compilers contain specialized transliterated
// variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
// for instance. If you make changes here, make sure to modify the
// interpreter, and both C1 and C2 fast-path inline locking code emission.
//
// -----------------------------------------------------------------------------
#ifdef DTRACE_ENABLED
// Only bother with this argument setup if dtrace is available
// TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
#define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
char* bytes = NULL; \
int len = 0; \
jlong jtid = SharedRuntime::get_java_tid(thread); \
Symbol* klassname = ((oop)(obj))->klass()->name(); \
if (klassname != NULL) { \
bytes = (char*)klassname->bytes(); \
len = klassname->utf8_length(); \
}
#define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
{ \
if (DTraceMonitorProbes) { \
DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
HOTSPOT_MONITOR_WAIT(jtid, \
(uintptr_t)(monitor), bytes, len, (millis)); \
} \
}
#define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
#define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
#define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
#define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
{ \
if (DTraceMonitorProbes) { \
DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
(uintptr_t)(monitor), bytes, len); \
} \
}
#else // ndef DTRACE_ENABLED
#define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
#define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
#endif // ndef DTRACE_ENABLED
// This exists only as a workaround of dtrace bug 6254741
int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
return 0;
}
#define NINFLATIONLOCKS 256
static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
// global list of blocks of monitors
PaddedObjectMonitor* ObjectSynchronizer::g_block_list = NULL;
bool volatile ObjectSynchronizer::_is_async_deflation_requested = false;
jlong ObjectSynchronizer::_last_async_deflation_time_ns = 0;
struct ObjectMonitorListGlobals {
char _pad_prefix[OM_CACHE_LINE_SIZE];
// These are highly shared list related variables.
// To avoid false-sharing they need to be the sole occupants of a cache line.
// Global ObjectMonitor free list. Newly allocated and deflated
// ObjectMonitors are prepended here.
ObjectMonitor* _free_list;
DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(ObjectMonitor*));
// Global ObjectMonitor in-use list. When a JavaThread is exiting,
// ObjectMonitors on its per-thread in-use list are prepended here.
ObjectMonitor* _in_use_list;
DEFINE_PAD_MINUS_SIZE(2, OM_CACHE_LINE_SIZE, sizeof(ObjectMonitor*));
// Global ObjectMonitor wait list. Deflated ObjectMonitors wait on
// this list until after a handshake or a safepoint for platforms
// that don't support handshakes. After the handshake or safepoint,
// the deflated ObjectMonitors are prepended to free_list.
ObjectMonitor* _wait_list;
DEFINE_PAD_MINUS_SIZE(3, OM_CACHE_LINE_SIZE, sizeof(ObjectMonitor*));
int _free_count; // # on free_list
DEFINE_PAD_MINUS_SIZE(4, OM_CACHE_LINE_SIZE, sizeof(int));
int _in_use_count; // # on in_use_list
DEFINE_PAD_MINUS_SIZE(5, OM_CACHE_LINE_SIZE, sizeof(int));
int _population; // # Extant -- in circulation
DEFINE_PAD_MINUS_SIZE(6, OM_CACHE_LINE_SIZE, sizeof(int));
int _wait_count; // # on wait_list
DEFINE_PAD_MINUS_SIZE(7, OM_CACHE_LINE_SIZE, sizeof(int));
};
static ObjectMonitorListGlobals om_list_globals;
#define CHAINMARKER (cast_to_oop<intptr_t>(-1))
// =====================> Spin-lock functions
// ObjectMonitors are not lockable outside of this file. We use spin-locks
// implemented using a bit in the _next_om field instead of the heavier
// weight locking mechanisms for faster list management.
#define OM_LOCK_BIT 0x1
// Return true if the ObjectMonitor is locked.
// Otherwise returns false.
static bool is_locked(ObjectMonitor* om) {
return ((intptr_t)om->next_om_acquire() & OM_LOCK_BIT) == OM_LOCK_BIT;
}
// Mark an ObjectMonitor* with OM_LOCK_BIT and return it.
static ObjectMonitor* mark_om_ptr(ObjectMonitor* om) {
return (ObjectMonitor*)((intptr_t)om | OM_LOCK_BIT);
}
// Return the unmarked next field in an ObjectMonitor. Note: the next
// field may or may not have been marked with OM_LOCK_BIT originally.
static ObjectMonitor* unmarked_next(ObjectMonitor* om) {
return (ObjectMonitor*)((intptr_t)om->next_om() & ~OM_LOCK_BIT);
}
// Try to lock an ObjectMonitor. Returns true if locking was successful.
// Otherwise returns false.
static bool try_om_lock(ObjectMonitor* om) {
// Get current next field without any OM_LOCK_BIT value.
ObjectMonitor* next = unmarked_next(om);
if (om->try_set_next_om(next, mark_om_ptr(next)) != next) {
return false; // Cannot lock the ObjectMonitor.
}
return true;
}
// Lock an ObjectMonitor.
static void om_lock(ObjectMonitor* om) {
while (true) {
if (try_om_lock(om)) {
return;
}
}
}
// Unlock an ObjectMonitor.
static void om_unlock(ObjectMonitor* om) {
ObjectMonitor* next = om->next_om();
guarantee(((intptr_t)next & OM_LOCK_BIT) == OM_LOCK_BIT, "next=" INTPTR_FORMAT
" must have OM_LOCK_BIT=%x set.", p2i(next), OM_LOCK_BIT);
next = (ObjectMonitor*)((intptr_t)next & ~OM_LOCK_BIT); // Clear OM_LOCK_BIT.
om->release_set_next_om(next);
}
// Get the list head after locking it. Returns the list head or NULL
// if the list is empty.
static ObjectMonitor* get_list_head_locked(ObjectMonitor** list_p) {
while (true) {
// Acquire semantics not needed on this list load since we're
// checking for NULL here or following up with a cmpxchg() via
// try_om_lock() below and we retry on cmpxchg() failure.
ObjectMonitor* mid = Atomic::load(list_p);
if (mid == NULL) {
return NULL; // The list is empty.
}
if (try_om_lock(mid)) {
// Acquire semantics not needed on this list load since memory is
// already consistent due to the cmpxchg() via try_om_lock() above.
if (Atomic::load(list_p) != mid) {
// The list head changed before we could lock it so we have to retry.
om_unlock(mid);
continue;
}
return mid;
}
}
}
#undef OM_LOCK_BIT
// =====================> List Management functions
// Prepend a list of ObjectMonitors to the specified *list_p. 'tail' is
// the last ObjectMonitor in the list and there are 'count' on the list.
// Also updates the specified *count_p.
static void prepend_list_to_common(ObjectMonitor* list, ObjectMonitor* tail,
int count, ObjectMonitor** list_p,
int* count_p) {
while (true) {
// Acquire semantics not needed on this list load since we're
// following up with a cmpxchg() via try_om_lock() below and we
// retry on cmpxchg() failure.
ObjectMonitor* cur = Atomic::load(list_p);
// Prepend list to *list_p.
if (!try_om_lock(tail)) {
// Failed to lock tail due to a list walker so try it all again.
continue;
}
// Release semantics not needed on this "unlock" since memory is
// already consistent due to the cmpxchg() via try_om_lock() above.
tail->set_next_om(cur); // tail now points to cur (and unlocks tail)
if (cur == NULL) {
// No potential race with takers or other prependers since
// *list_p is empty.
if (Atomic::cmpxchg(list_p, cur, list) == cur) {
// Successfully switched *list_p to the list value.
Atomic::add(count_p, count);
break;
}
// Implied else: try it all again
} else {
if (!try_om_lock(cur)) {
continue; // failed to lock cur so try it all again
}
// We locked cur so try to switch *list_p to the list value.
if (Atomic::cmpxchg(list_p, cur, list) != cur) {
// The list head has changed so unlock cur and try again:
om_unlock(cur);
continue;
}
Atomic::add(count_p, count);
om_unlock(cur);
break;
}
}
}
// Prepend a newly allocated block of ObjectMonitors to g_block_list and
// om_list_globals._free_list. Also updates om_list_globals._population
// and om_list_globals._free_count.
void ObjectSynchronizer::prepend_block_to_lists(PaddedObjectMonitor* new_blk) {
// First we handle g_block_list:
while (true) {
PaddedObjectMonitor* cur = Atomic::load(&g_block_list);
// Prepend new_blk to g_block_list. The first ObjectMonitor in
// a block is reserved for use as linkage to the next block.
new_blk[0].set_next_om(cur);
if (Atomic::cmpxchg(&g_block_list, cur, new_blk) == cur) {
// Successfully switched g_block_list to the new_blk value.
Atomic::add(&om_list_globals._population, _BLOCKSIZE - 1);
break;
}
// Implied else: try it all again
}
// Second we handle om_list_globals._free_list:
prepend_list_to_common(new_blk + 1, &new_blk[_BLOCKSIZE - 1], _BLOCKSIZE - 1,
&om_list_globals._free_list, &om_list_globals._free_count);
}
// Prepend a list of ObjectMonitors to om_list_globals._free_list.
// 'tail' is the last ObjectMonitor in the list and there are 'count'
// on the list. Also updates om_list_globals._free_count.
static void prepend_list_to_global_free_list(ObjectMonitor* list,
ObjectMonitor* tail, int count) {
prepend_list_to_common(list, tail, count, &om_list_globals._free_list,
&om_list_globals._free_count);
}
// Prepend a list of ObjectMonitors to om_list_globals._wait_list.
// 'tail' is the last ObjectMonitor in the list and there are 'count'
// on the list. Also updates om_list_globals._wait_count.
static void prepend_list_to_global_wait_list(ObjectMonitor* list,
ObjectMonitor* tail, int count) {
prepend_list_to_common(list, tail, count, &om_list_globals._wait_list,
&om_list_globals._wait_count);
}
// Prepend a list of ObjectMonitors to om_list_globals._in_use_list.
// 'tail' is the last ObjectMonitor in the list and there are 'count'
// on the list. Also updates om_list_globals._in_use_list.
static void prepend_list_to_global_in_use_list(ObjectMonitor* list,
ObjectMonitor* tail, int count) {
prepend_list_to_common(list, tail, count, &om_list_globals._in_use_list,
&om_list_globals._in_use_count);
}
// Prepend an ObjectMonitor to the specified list. Also updates
// the specified counter.
static void prepend_to_common(ObjectMonitor* m, ObjectMonitor** list_p,
int* count_p) {
while (true) {
om_lock(m); // Lock m so we can safely update its next field.
ObjectMonitor* cur = NULL;
// Lock the list head to guard against races with a list walker
// or async deflater thread (which only races in om_in_use_list):
if ((cur = get_list_head_locked(list_p)) != NULL) {
// List head is now locked so we can safely switch it. Release
// semantics not needed on this "unlock" since memory is already
// consistent due to the cmpxchg() via get_list_head_locked() above.
m->set_next_om(cur); // m now points to cur (and unlocks m)
OrderAccess::storestore(); // Make sure set_next_om() is seen first.
Atomic::store(list_p, m); // Switch list head to unlocked m.
om_unlock(cur);
break;
}
// The list is empty so try to set the list head.
assert(cur == NULL, "cur must be NULL: cur=" INTPTR_FORMAT, p2i(cur));
// Release semantics not needed on this "unlock" since memory
// is already consistent.
m->set_next_om(cur); // m now points to NULL (and unlocks m)
if (Atomic::cmpxchg(list_p, cur, m) == cur) {
// List head is now unlocked m.
break;
}
// Implied else: try it all again
}
Atomic::inc(count_p);
}
// Prepend an ObjectMonitor to a per-thread om_free_list.
// Also updates the per-thread om_free_count.
static void prepend_to_om_free_list(Thread* self, ObjectMonitor* m) {
prepend_to_common(m, &self->om_free_list, &self->om_free_count);
}
// Prepend an ObjectMonitor to a per-thread om_in_use_list.
// Also updates the per-thread om_in_use_count.
static void prepend_to_om_in_use_list(Thread* self, ObjectMonitor* m) {
prepend_to_common(m, &self->om_in_use_list, &self->om_in_use_count);
}
// Take an ObjectMonitor from the start of the specified list. Also
// decrements the specified counter. Returns NULL if none are available.
static ObjectMonitor* take_from_start_of_common(ObjectMonitor** list_p,
int* count_p) {
ObjectMonitor* take = NULL;
// Lock the list head to guard against races with a list walker
// or async deflater thread (which only races in om_list_globals._free_list):
if ((take = get_list_head_locked(list_p)) == NULL) {
return NULL; // None are available.
}
ObjectMonitor* next = unmarked_next(take);
// Switch locked list head to next (which unlocks the list head, but
// leaves take locked). Release semantics not needed on this "unlock"
// since memory is already consistent due to the cmpxchg() via
// get_list_head_locked() above.
Atomic::store(list_p, next);
Atomic::dec(count_p);
// Unlock take, but leave the next value for any lagging list
// walkers. It will get cleaned up when take is prepended to
// the in-use list:
om_unlock(take);
return take;
}
// Take an ObjectMonitor from the start of the om_list_globals._free_list.
// Also updates om_list_globals._free_count. Returns NULL if none are
// available.
static ObjectMonitor* take_from_start_of_global_free_list() {
return take_from_start_of_common(&om_list_globals._free_list,
&om_list_globals._free_count);
}
// Take an ObjectMonitor from the start of a per-thread free-list.
// Also updates om_free_count. Returns NULL if none are available.
static ObjectMonitor* take_from_start_of_om_free_list(Thread* self) {
return take_from_start_of_common(&self->om_free_list, &self->om_free_count);
}
// =====================> Quick functions
// The quick_* forms are special fast-path variants used to improve
// performance. In the simplest case, a "quick_*" implementation could
// simply return false, in which case the caller will perform the necessary
// state transitions and call the slow-path form.
// The fast-path is designed to handle frequently arising cases in an efficient
// manner and is just a degenerate "optimistic" variant of the slow-path.
// returns true -- to indicate the call was satisfied.
// returns false -- to indicate the call needs the services of the slow-path.
// A no-loitering ordinance is in effect for code in the quick_* family
// operators: safepoints or indefinite blocking (blocking that might span a
// safepoint) are forbidden. Generally the thread_state() is _in_Java upon
// entry.
//
// Consider: An interesting optimization is to have the JIT recognize the
// following common idiom:
// synchronized (someobj) { .... ; notify(); }
// That is, we find a notify() or notifyAll() call that immediately precedes
// the monitorexit operation. In that case the JIT could fuse the operations
// into a single notifyAndExit() runtime primitive.
bool ObjectSynchronizer::quick_notify(oopDesc* obj, Thread* self, bool all) {
assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(self->is_Java_thread(), "invariant");
assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
NoSafepointVerifier nsv;
if (obj == NULL) return false; // slow-path for invalid obj
const markWord mark = obj->mark();
if (mark.has_locker() && self->is_lock_owned((address)mark.locker())) {
// Degenerate notify
// stack-locked by caller so by definition the implied waitset is empty.
return true;
}
if (mark.has_monitor()) {
ObjectMonitor* const mon = mark.monitor();
assert(mon->object() == obj, "invariant");
if (mon->owner() != self) return false; // slow-path for IMS exception
if (mon->first_waiter() != NULL) {
// We have one or more waiters. Since this is an inflated monitor
// that we own, we can transfer one or more threads from the waitset
// to the entrylist here and now, avoiding the slow-path.
if (all) {
DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self);
} else {
DTRACE_MONITOR_PROBE(notify, mon, obj, self);
}
int free_count = 0;
do {
mon->INotify(self);
++free_count;
} while (mon->first_waiter() != NULL && all);
OM_PERFDATA_OP(Notifications, inc(free_count));
}
return true;
}
// biased locking and any other IMS exception states take the slow-path
return false;
}
// The LockNode emitted directly at the synchronization site would have
// been too big if it were to have included support for the cases of inflated
// recursive enter and exit, so they go here instead.
// Note that we can't safely call AsyncPrintJavaStack() from within
// quick_enter() as our thread state remains _in_Java.
bool ObjectSynchronizer::quick_enter(oop obj, Thread* self,
BasicLock * lock) {
assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(self->is_Java_thread(), "invariant");
assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
NoSafepointVerifier nsv;
if (obj == NULL) return false; // Need to throw NPE
const markWord mark = obj->mark();
if (mark.has_monitor()) {
ObjectMonitor* const m = mark.monitor();
// An async deflation can race us before we manage to make the
// ObjectMonitor busy by setting the owner below. If we detect
// that race we just bail out to the slow-path here.
if (m->object() == NULL) {
return false;
}
Thread* const owner = (Thread *) m->_owner;
// Lock contention and Transactional Lock Elision (TLE) diagnostics
// and observability
// Case: light contention possibly amenable to TLE
// Case: TLE inimical operations such as nested/recursive synchronization
if (owner == self) {
m->_recursions++;
return true;
}
// This Java Monitor is inflated so obj's header will never be
// displaced to this thread's BasicLock. Make the displaced header
// non-NULL so this BasicLock is not seen as recursive nor as
// being locked. We do this unconditionally so that this thread's
// BasicLock cannot be mis-interpreted by any stack walkers. For
// performance reasons, stack walkers generally first check for
// Biased Locking in the object's header, the second check is for
// stack-locking in the object's header, the third check is for
// recursive stack-locking in the displaced header in the BasicLock,
// and last are the inflated Java Monitor (ObjectMonitor) checks.
lock->set_displaced_header(markWord::unused_mark());
if (owner == NULL && m->try_set_owner_from(NULL, self) == NULL) {
assert(m->_recursions == 0, "invariant");
return true;
}
}
// Note that we could inflate in quick_enter.
// This is likely a useful optimization
// Critically, in quick_enter() we must not:
// -- perform bias revocation, or
// -- block indefinitely, or
// -- reach a safepoint
return false; // revert to slow-path
}
// -----------------------------------------------------------------------------
// Monitor Enter/Exit
// The interpreter and compiler assembly code tries to lock using the fast path
// of this algorithm. Make sure to update that code if the following function is
// changed. The implementation is extremely sensitive to race condition. Be careful.
void ObjectSynchronizer::enter(Handle obj, BasicLock* lock, TRAPS) {
if (UseBiasedLocking) {
if (!SafepointSynchronize::is_at_safepoint()) {
BiasedLocking::revoke(obj, THREAD);
} else {
BiasedLocking::revoke_at_safepoint(obj);
}
}
markWord mark = obj->mark();
assert(!mark.has_bias_pattern(), "should not see bias pattern here");
if (mark.is_neutral()) {
// Anticipate successful CAS -- the ST of the displaced mark must
// be visible <= the ST performed by the CAS.
lock->set_displaced_header(mark);
if (mark == obj()->cas_set_mark(markWord::from_pointer(lock), mark)) {
return;
}
// Fall through to inflate() ...
} else if (mark.has_locker() &&
THREAD->is_lock_owned((address)mark.locker())) {
assert(lock != mark.locker(), "must not re-lock the same lock");
assert(lock != (BasicLock*)obj->mark().value(), "don't relock with same BasicLock");
lock->set_displaced_header(markWord::from_pointer(NULL));
return;
}
// The object header will never be displaced to this lock,
// so it does not matter what the value is, except that it
// must be non-zero to avoid looking like a re-entrant lock,
// and must not look locked either.
lock->set_displaced_header(markWord::unused_mark());
// An async deflation can race after the inflate() call and before
// enter() can make the ObjectMonitor busy. enter() returns false if
// we have lost the race to async deflation and we simply try again.
while (true) {
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_monitor_enter);
if (monitor->enter(THREAD)) {
return;
}
}
}
void ObjectSynchronizer::exit(oop object, BasicLock* lock, TRAPS) {
markWord mark = object->mark();
// We cannot check for Biased Locking if we are racing an inflation.
assert(mark == markWord::INFLATING() ||
!mark.has_bias_pattern(), "should not see bias pattern here");
markWord dhw = lock->displaced_header();
if (dhw.value() == 0) {
// If the displaced header is NULL, then this exit matches up with
// a recursive enter. No real work to do here except for diagnostics.
#ifndef PRODUCT
if (mark != markWord::INFLATING()) {
// Only do diagnostics if we are not racing an inflation. Simply
// exiting a recursive enter of a Java Monitor that is being
// inflated is safe; see the has_monitor() comment below.
assert(!mark.is_neutral(), "invariant");
assert(!mark.has_locker() ||
THREAD->is_lock_owned((address)mark.locker()), "invariant");
if (mark.has_monitor()) {
// The BasicLock's displaced_header is marked as a recursive
// enter and we have an inflated Java Monitor (ObjectMonitor).
// This is a special case where the Java Monitor was inflated
// after this thread entered the stack-lock recursively. When a
// Java Monitor is inflated, we cannot safely walk the Java
// Monitor owner's stack and update the BasicLocks because a
// Java Monitor can be asynchronously inflated by a thread that
// does not own the Java Monitor.
ObjectMonitor* m = mark.monitor();
assert(((oop)(m->object()))->mark() == mark, "invariant");
assert(m->is_entered(THREAD), "invariant");
}
}
#endif
return;
}
if (mark == markWord::from_pointer(lock)) {
// If the object is stack-locked by the current thread, try to
// swing the displaced header from the BasicLock back to the mark.
assert(dhw.is_neutral(), "invariant");
if (object->cas_set_mark(dhw, mark) == mark) {
return;
}
}
// We have to take the slow-path of possible inflation and then exit.
// The ObjectMonitor* can't be async deflated until ownership is
// dropped inside exit() and the ObjectMonitor* must be !is_busy().
ObjectMonitor* monitor = inflate(THREAD, object, inflate_cause_vm_internal);
monitor->exit(true, THREAD);
}
// -----------------------------------------------------------------------------
// Class Loader support to workaround deadlocks on the class loader lock objects
// Also used by GC
// complete_exit()/reenter() are used to wait on a nested lock
// i.e. to give up an outer lock completely and then re-enter
// Used when holding nested locks - lock acquisition order: lock1 then lock2
// 1) complete_exit lock1 - saving recursion count
// 2) wait on lock2
// 3) when notified on lock2, unlock lock2
// 4) reenter lock1 with original recursion count
// 5) lock lock2
// NOTE: must use heavy weight monitor to handle complete_exit/reenter()
intx ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke(obj, THREAD);
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
// The ObjectMonitor* can't be async deflated until ownership is
// dropped inside exit() and the ObjectMonitor* must be !is_busy().
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_vm_internal);
intptr_t ret_code = monitor->complete_exit(THREAD);
return ret_code;
}
// NOTE: must use heavy weight monitor to handle complete_exit/reenter()
void ObjectSynchronizer::reenter(Handle obj, intx recursions, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke(obj, THREAD);
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
// An async deflation can race after the inflate() call and before
// reenter() -> enter() can make the ObjectMonitor busy. reenter() ->
// enter() returns false if we have lost the race to async deflation
// and we simply try again.
while (true) {
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_vm_internal);
if (monitor->reenter(recursions, THREAD)) {
return;
}
}
}
// -----------------------------------------------------------------------------
// JNI locks on java objects
// NOTE: must use heavy weight monitor to handle jni monitor enter
void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
// the current locking is from JNI instead of Java code
if (UseBiasedLocking) {
BiasedLocking::revoke(obj, THREAD);
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
THREAD->set_current_pending_monitor_is_from_java(false);
// An async deflation can race after the inflate() call and before
// enter() can make the ObjectMonitor busy. enter() returns false if
// we have lost the race to async deflation and we simply try again.
while (true) {
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_jni_enter);
if (monitor->enter(THREAD)) {
break;
}
}
THREAD->set_current_pending_monitor_is_from_java(true);
}
// NOTE: must use heavy weight monitor to handle jni monitor exit
void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
if (UseBiasedLocking) {
Handle h_obj(THREAD, obj);
BiasedLocking::revoke(h_obj, THREAD);
obj = h_obj();
}
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
// The ObjectMonitor* can't be async deflated until ownership is
// dropped inside exit() and the ObjectMonitor* must be !is_busy().
ObjectMonitor* monitor = inflate(THREAD, obj, inflate_cause_jni_exit);
// If this thread has locked the object, exit the monitor. We
// intentionally do not use CHECK here because we must exit the
// monitor even if an exception is pending.
if (monitor->check_owner(THREAD)) {
monitor->exit(true, THREAD);
}
}
// -----------------------------------------------------------------------------
// Internal VM locks on java objects
// standard constructor, allows locking failures
ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool do_lock) {
_dolock = do_lock;
_thread = thread;
_thread->check_for_valid_safepoint_state();
_obj = obj;
if (_dolock) {
ObjectSynchronizer::enter(_obj, &_lock, _thread);
}
}
ObjectLocker::~ObjectLocker() {
if (_dolock) {
ObjectSynchronizer::exit(_obj(), &_lock, _thread);
}
}
// -----------------------------------------------------------------------------
// Wait/Notify/NotifyAll
// NOTE: must use heavy weight monitor to handle wait()
int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke(obj, THREAD);
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
if (millis < 0) {
THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
// The ObjectMonitor* can't be async deflated because the _waiters
// field is incremented before ownership is dropped and decremented
// after ownership is regained.
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_wait);
DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
monitor->wait(millis, true, THREAD);
// This dummy call is in place to get around dtrace bug 6254741. Once
// that's fixed we can uncomment the following line, remove the call
// and change this function back into a "void" func.
// DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
int ret_code = dtrace_waited_probe(monitor, obj, THREAD);
return ret_code;
}
void ObjectSynchronizer::wait_uninterruptibly(Handle obj, jlong millis, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke(obj, THREAD);
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
if (millis < 0) {
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
// The ObjectMonitor* can't be async deflated because the _waiters
// field is incremented before ownership is dropped and decremented
// after ownership is regained.
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_wait);
monitor->wait(millis, false, THREAD);
}
void ObjectSynchronizer::notify(Handle obj, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke(obj, THREAD);
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
markWord mark = obj->mark();
if (mark.has_locker() && THREAD->is_lock_owned((address)mark.locker())) {
return;
}
// The ObjectMonitor* can't be async deflated until ownership is
// dropped by the calling thread.
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_notify);
monitor->notify(THREAD);
}
// NOTE: see comment of notify()
void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke(obj, THREAD);
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
markWord mark = obj->mark();
if (mark.has_locker() && THREAD->is_lock_owned((address)mark.locker())) {
return;
}
// The ObjectMonitor* can't be async deflated until ownership is
// dropped by the calling thread.
ObjectMonitor* monitor = inflate(THREAD, obj(), inflate_cause_notify);
monitor->notifyAll(THREAD);
}
// -----------------------------------------------------------------------------
// Hash Code handling
//
// Performance concern:
// OrderAccess::storestore() calls release() which at one time stored 0
// into the global volatile OrderAccess::dummy variable. This store was
// unnecessary for correctness. Many threads storing into a common location
// causes considerable cache migration or "sloshing" on large SMP systems.
// As such, I avoided using OrderAccess::storestore(). In some cases
// OrderAccess::fence() -- which incurs local latency on the executing
// processor -- is a better choice as it scales on SMP systems.
//
// See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
// a discussion of coherency costs. Note that all our current reference
// platforms provide strong ST-ST order, so the issue is moot on IA32,
// x64, and SPARC.
//
// As a general policy we use "volatile" to control compiler-based reordering
// and explicit fences (barriers) to control for architectural reordering
// performed by the CPU(s) or platform.
struct SharedGlobals {
char _pad_prefix[OM_CACHE_LINE_SIZE];
// These are highly shared mostly-read variables.
// To avoid false-sharing they need to be the sole occupants of a cache line.
volatile int stw_random;
volatile int stw_cycle;
DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
// Hot RW variable -- Sequester to avoid false-sharing
volatile int hc_sequence;
DEFINE_PAD_MINUS_SIZE(2, OM_CACHE_LINE_SIZE, sizeof(volatile int));
};
static SharedGlobals GVars;
static markWord read_stable_mark(oop obj) {
markWord mark = obj->mark();
if (!mark.is_being_inflated()) {
return mark; // normal fast-path return
}
int its = 0;
for (;;) {
markWord mark = obj->mark();
if (!mark.is_being_inflated()) {
return mark; // normal fast-path return
}
// The object is being inflated by some other thread.
// The caller of read_stable_mark() must wait for inflation to complete.
// Avoid live-lock
// TODO: consider calling SafepointSynchronize::do_call_back() while
// spinning to see if there's a safepoint pending. If so, immediately
// yielding or blocking would be appropriate. Avoid spinning while
// there is a safepoint pending.
// TODO: add inflation contention performance counters.
// TODO: restrict the aggregate number of spinners.
++its;
if (its > 10000 || !os::is_MP()) {
if (its & 1) {
os::naked_yield();
} else {
// Note that the following code attenuates the livelock problem but is not
// a complete remedy. A more complete solution would require that the inflating
// thread hold the associated inflation lock. The following code simply restricts
// the number of spinners to at most one. We'll have N-2 threads blocked
// on the inflationlock, 1 thread holding the inflation lock and using
// a yield/park strategy, and 1 thread in the midst of inflation.
// A more refined approach would be to change the encoding of INFLATING
// to allow encapsulation of a native thread pointer. Threads waiting for
// inflation to complete would use CAS to push themselves onto a singly linked
// list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
// and calling park(). When inflation was complete the thread that accomplished inflation
// would detach the list and set the markword to inflated with a single CAS and
// then for each thread on the list, set the flag and unpark() the thread.
// This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
// wakes at most one thread whereas we need to wake the entire list.
int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
int YieldThenBlock = 0;
assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
while (obj->mark() == markWord::INFLATING()) {
// Beware: NakedYield() is advisory and has almost no effect on some platforms
// so we periodically call self->_ParkEvent->park(1).
// We use a mixed spin/yield/block mechanism.
if ((YieldThenBlock++) >= 16) {
Thread::current()->_ParkEvent->park(1);
} else {
os::naked_yield();
}
}
Thread::muxRelease(gInflationLocks + ix);
}
} else {
SpinPause(); // SMP-polite spinning
}
}
}
// hashCode() generation :
//
// Possibilities:
// * MD5Digest of {obj,stw_random}
// * CRC32 of {obj,stw_random} or any linear-feedback shift register function.
// * A DES- or AES-style SBox[] mechanism
// * One of the Phi-based schemes, such as:
// 2654435761 = 2^32 * Phi (golden ratio)
// HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stw_random ;
// * A variation of Marsaglia's shift-xor RNG scheme.
// * (obj ^ stw_random) is appealing, but can result
// in undesirable regularity in the hashCode values of adjacent objects
// (objects allocated back-to-back, in particular). This could potentially
// result in hashtable collisions and reduced hashtable efficiency.
// There are simple ways to "diffuse" the middle address bits over the
// generated hashCode values:
static inline intptr_t get_next_hash(Thread* self, oop obj) {
intptr_t value = 0;
if (hashCode == 0) {
// This form uses global Park-Miller RNG.
// On MP system we'll have lots of RW access to a global, so the
// mechanism induces lots of coherency traffic.
value = os::random();
} else if (hashCode == 1) {
// This variation has the property of being stable (idempotent)
// between STW operations. This can be useful in some of the 1-0
// synchronization schemes.
intptr_t addr_bits = cast_from_oop<intptr_t>(obj) >> 3;
value = addr_bits ^ (addr_bits >> 5) ^ GVars.stw_random;
} else if (hashCode == 2) {
value = 1; // for sensitivity testing
} else if (hashCode == 3) {
value = ++GVars.hc_sequence;
} else if (hashCode == 4) {
value = cast_from_oop<intptr_t>(obj);
} else {
// Marsaglia's xor-shift scheme with thread-specific state
// This is probably the best overall implementation -- we'll
// likely make this the default in future releases.
unsigned t = self->_hashStateX;
t ^= (t << 11);
self->_hashStateX = self->_hashStateY;
self->_hashStateY = self->_hashStateZ;
self->_hashStateZ = self->_hashStateW;
unsigned v = self->_hashStateW;
v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
self->_hashStateW = v;
value = v;
}
value &= markWord::hash_mask;
if (value == 0) value = 0xBAD;
assert(value != markWord::no_hash, "invariant");
return value;
}
intptr_t ObjectSynchronizer::FastHashCode(Thread* self, oop obj) {
if (UseBiasedLocking) {
// NOTE: many places throughout the JVM do not expect a safepoint
// to be taken here. However, we only ever bias Java instances and all
// of the call sites of identity_hash that might revoke biases have
// been checked to make sure they can handle a safepoint. The
// added check of the bias pattern is to avoid useless calls to
// thread-local storage.
if (obj->mark().has_bias_pattern()) {
// Handle for oop obj in case of STW safepoint
Handle hobj(self, obj);
// Relaxing assertion for bug 6320749.
assert(Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(),
"biases should not be seen by VM thread here");
BiasedLocking::revoke(hobj, JavaThread::current());
obj = hobj();
assert(!obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
}
while (true) {
ObjectMonitor* monitor = NULL;
markWord temp, test;
intptr_t hash;
markWord mark = read_stable_mark(obj);
// object should remain ineligible for biased locking
assert(!mark.has_bias_pattern(), "invariant");
if (mark.is_neutral()) { // if this is a normal header
hash = mark.hash();
if (hash != 0) { // if it has a hash, just return it
return hash;
}
hash = get_next_hash(self, obj); // get a new hash
temp = mark.copy_set_hash(hash); // merge the hash into header
// try to install the hash
test = obj->cas_set_mark(temp, mark);
if (test == mark) { // if the hash was installed, return it
return hash;
}
// Failed to install the hash. It could be that another thread
// installed the hash just before our attempt or inflation has
// occurred or... so we fall thru to inflate the monitor for
// stability and then install the hash.
} else if (mark.has_monitor()) {
monitor = mark.monitor();
temp = monitor->header();
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
hash = temp.hash();
if (hash != 0) {
// It has a hash.
// Separate load of dmw/header above from the loads in
// is_being_async_deflated().
if (support_IRIW_for_not_multiple_copy_atomic_cpu) {
// A non-multiple copy atomic (nMCA) machine needs a bigger
// hammer to separate the load above and the loads below.
OrderAccess::fence();
} else {
OrderAccess::loadload();
}
if (monitor->is_being_async_deflated()) {
// But we can't safely use the hash if we detect that async
// deflation has occurred. So we attempt to restore the
// header/dmw to the object's header so that we only retry
// once if the deflater thread happens to be slow.
monitor->install_displaced_markword_in_object(obj);
continue;
}
return hash;
}
// Fall thru so we only have one place that installs the hash in
// the ObjectMonitor.
} else if (self->is_lock_owned((address)mark.locker())) {
// This is a stack lock owned by the calling thread so fetch the
// displaced markWord from the BasicLock on the stack.
temp = mark.displaced_mark_helper();
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
hash = temp.hash();
if (hash != 0) { // if it has a hash, just return it
return hash;
}
// WARNING:
// The displaced header in the BasicLock on a thread's stack
// is strictly immutable. It CANNOT be changed in ANY cases.
// So we have to inflate the stack lock into an ObjectMonitor
// even if the current thread owns the lock. The BasicLock on
// a thread's stack can be asynchronously read by other threads
// during an inflate() call so any change to that stack memory
// may not propagate to other threads correctly.
}
// Inflate the monitor to set the hash.
// An async deflation can race after the inflate() call and before we
// can update the ObjectMonitor's header with the hash value below.
monitor = inflate(self, obj, inflate_cause_hash_code);
// Load ObjectMonitor's header/dmw field and see if it has a hash.
mark = monitor->header();
assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value());
hash = mark.hash();
if (hash == 0) { // if it does not have a hash
hash = get_next_hash(self, obj); // get a new hash
temp = mark.copy_set_hash(hash); // merge the hash into header
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
uintptr_t v = Atomic::cmpxchg((volatile uintptr_t*)monitor->header_addr(), mark.value(), temp.value());
test = markWord(v);
if (test != mark) {
// The attempt to update the ObjectMonitor's header/dmw field
// did not work. This can happen if another thread managed to
// merge in the hash just before our cmpxchg().
// If we add any new usages of the header/dmw field, this code
// will need to be updated.
hash = test.hash();
assert(test.is_neutral(), "invariant: header=" INTPTR_FORMAT, test.value());
assert(hash != 0, "should only have lost the race to a thread that set a non-zero hash");
}
if (monitor->is_being_async_deflated()) {
// If we detect that async deflation has occurred, then we
// attempt to restore the header/dmw to the object's header
// so that we only retry once if the deflater thread happens
// to be slow.
monitor->install_displaced_markword_in_object(obj);
continue;
}
}
// We finally get the hash.
return hash;
}
}
// Deprecated -- use FastHashCode() instead.
intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
return FastHashCode(Thread::current(), obj());
}
bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
Handle h_obj) {
if (UseBiasedLocking) {
BiasedLocking::revoke(h_obj, thread);
assert(!h_obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
assert(thread == JavaThread::current(), "Can only be called on current thread");
oop obj = h_obj();
markWord mark = read_stable_mark(obj);
// Uncontended case, header points to stack
if (mark.has_locker()) {
return thread->is_lock_owned((address)mark.locker());
}
// Contended case, header points to ObjectMonitor (tagged pointer)
if (mark.has_monitor()) {
// The first stage of async deflation does not affect any field
// used by this comparison so the ObjectMonitor* is usable here.
ObjectMonitor* monitor = mark.monitor();
return monitor->is_entered(thread) != 0;
}
// Unlocked case, header in place
assert(mark.is_neutral(), "sanity check");
return false;
}
// Be aware of this method could revoke bias of the lock object.
// This method queries the ownership of the lock handle specified by 'h_obj'.
// If the current thread owns the lock, it returns owner_self. If no
// thread owns the lock, it returns owner_none. Otherwise, it will return
// owner_other.
ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
(JavaThread *self, Handle h_obj) {
// The caller must beware this method can revoke bias, and
// revocation can result in a safepoint.
assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(self->thread_state() != _thread_blocked, "invariant");
// Possible mark states: neutral, biased, stack-locked, inflated
if (UseBiasedLocking && h_obj()->mark().has_bias_pattern()) {
// CASE: biased
BiasedLocking::revoke(h_obj, self);
assert(!h_obj->mark().has_bias_pattern(),
"biases should be revoked by now");
}
assert(self == JavaThread::current(), "Can only be called on current thread");
oop obj = h_obj();
markWord mark = read_stable_mark(obj);
// CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
if (mark.has_locker()) {
return self->is_lock_owned((address)mark.locker()) ?
owner_self : owner_other;
}
// CASE: inflated. Mark (tagged pointer) points to an ObjectMonitor.
if (mark.has_monitor()) {
// The first stage of async deflation does not affect any field
// used by this comparison so the ObjectMonitor* is usable here.
ObjectMonitor* monitor = mark.monitor();
void* owner = monitor->owner();
if (owner == NULL) return owner_none;
return (owner == self ||
self->is_lock_owned((address)owner)) ? owner_self : owner_other;
}
// CASE: neutral
assert(mark.is_neutral(), "sanity check");
return owner_none; // it's unlocked
}
// FIXME: jvmti should call this
JavaThread* ObjectSynchronizer::get_lock_owner(ThreadsList * t_list, Handle h_obj) {
if (UseBiasedLocking) {
if (SafepointSynchronize::is_at_safepoint()) {
BiasedLocking::revoke_at_safepoint(h_obj);
} else {
BiasedLocking::revoke(h_obj, JavaThread::current());
}
assert(!h_obj->mark().has_bias_pattern(), "biases should be revoked by now");
}
oop obj = h_obj();
address owner = NULL;
markWord mark = read_stable_mark(obj);
// Uncontended case, header points to stack
if (mark.has_locker()) {
owner = (address) mark.locker();
}
// Contended case, header points to ObjectMonitor (tagged pointer)
else if (mark.has_monitor()) {
// The first stage of async deflation does not affect any field
// used by this comparison so the ObjectMonitor* is usable here.
ObjectMonitor* monitor = mark.monitor();
assert(monitor != NULL, "monitor should be non-null");
owner = (address) monitor->owner();
}
if (owner != NULL) {
// owning_thread_from_monitor_owner() may also return NULL here
return Threads::owning_thread_from_monitor_owner(t_list, owner);
}
// Unlocked case, header in place
// Cannot have assertion since this object may have been
// locked by another thread when reaching here.
// assert(mark.is_neutral(), "sanity check");
return NULL;
}
// Visitors ...
void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
PaddedObjectMonitor* block = Atomic::load(&g_block_list);
while (block != NULL) {
assert(block->object() == CHAINMARKER, "must be a block header");
for (int i = _BLOCKSIZE - 1; i > 0; i--) {
ObjectMonitor* mid = (ObjectMonitor *)(block + i);
if (mid->object() != NULL) {
// Only process with closure if the object is set.
// monitors_iterate() is only called at a safepoint or when the
// target thread is suspended or when the target thread is
// operating on itself. The current closures in use today are
// only interested in an owned ObjectMonitor and ownership
// cannot be dropped under the calling contexts so the
// ObjectMonitor cannot be async deflated.
closure->do_monitor(mid);
}
}
// unmarked_next() is not needed with g_block_list (no locking
// used with block linkage _next_om fields).
block = (PaddedObjectMonitor*)block->next_om();
}
}
static bool monitors_used_above_threshold() {
int population = Atomic::load(&om_list_globals._population);
if (population == 0) {
return false;
}
if (MonitorUsedDeflationThreshold > 0) {
int monitors_used = population - Atomic::load(&om_list_globals._free_count) -
Atomic::load(&om_list_globals._wait_count);
int monitor_usage = (monitors_used * 100LL) / population;
return monitor_usage > MonitorUsedDeflationThreshold;
}
return false;
}
bool ObjectSynchronizer::is_async_deflation_needed() {
if (is_async_deflation_requested()) {
// Async deflation request.
return true;
}
if (AsyncDeflationInterval > 0 &&
time_since_last_async_deflation_ms() > AsyncDeflationInterval &&
monitors_used_above_threshold()) {
// It's been longer than our specified deflate interval and there
// are too many monitors in use. We don't deflate more frequently
// than AsyncDeflationInterval (unless is_async_deflation_requested)
// in order to not swamp the ServiceThread.
return true;
}
return false;
}
bool ObjectSynchronizer::request_deflate_idle_monitors() {
bool is_JavaThread = Thread::current()->is_Java_thread();
bool ret_code = false;
jlong last_time = last_async_deflation_time_ns();
set_is_async_deflation_requested(true);
{
MonitorLocker ml(Service_lock, Mutex::_no_safepoint_check_flag);
ml.notify_all();
}
const int N_CHECKS = 5;
for (int i = 0; i < N_CHECKS; i++) { // sleep for at most 5 seconds
if (last_async_deflation_time_ns() > last_time) {
log_info(monitorinflation)("Async Deflation happened after %d check(s).", i);
ret_code = true;
break;
}
if (is_JavaThread) {
// JavaThread has to honor the blocking protocol.
ThreadBlockInVM tbivm(JavaThread::current());
os::naked_short_sleep(999); // sleep for almost 1 second
} else {
os::naked_short_sleep(999); // sleep for almost 1 second
}
}
if (!ret_code) {
log_info(monitorinflation)("Async Deflation DID NOT happen after %d checks.", N_CHECKS);
}
return ret_code;
}
jlong ObjectSynchronizer::time_since_last_async_deflation_ms() {
return (os::javaTimeNanos() - last_async_deflation_time_ns()) / (NANOUNITS / MILLIUNITS);
}
void ObjectSynchronizer::oops_do(OopClosure* f) {
// We only scan the global used list here (for moribund threads), and
// the thread-local monitors in Thread::oops_do().
global_used_oops_do(f);
}
void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
// Acquire semantics not needed since we're at a safepoint.
list_oops_do(Atomic::load(&om_list_globals._in_use_list), f);
}
void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
list_oops_do(thread->om_in_use_list, f);
}
void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
// The oops_do() phase does not overlap with monitor deflation
// so no need to lock ObjectMonitors for the list traversal.
for (ObjectMonitor* mid = list; mid != NULL; mid = unmarked_next(mid)) {
if (mid->object() != NULL) {
f->do_oop((oop*)mid->object_addr());
}
}
}
// -----------------------------------------------------------------------------
// ObjectMonitor Lifecycle
// -----------------------
// Inflation unlinks monitors from om_list_globals._free_list or a per-thread
// free list and associates them with objects. Async deflation disassociates
// idle monitors from objects. Such scavenged monitors are returned to the
// om_list_globals._free_list.
//
// ObjectMonitors reside in type-stable memory (TSM) and are immortal.
//
// Lifecycle:
// -- unassigned and on the om_list_globals._free_list
// -- unassigned and on a per-thread free list
// -- assigned to an object. The object is inflated and the mark refers
// to the ObjectMonitor.
ObjectMonitor* ObjectSynchronizer::om_alloc(Thread* self) {
// A large MAXPRIVATE value reduces both list lock contention
// and list coherency traffic, but also tends to increase the
// number of ObjectMonitors in circulation as well as the
// scavenge costs. As usual, we lean toward time in space-time
// tradeoffs.
const int MAXPRIVATE = 1024;
NoSafepointVerifier nsv;
for (;;) {
ObjectMonitor* m;
// 1: try to allocate from the thread's local om_free_list.
// Threads will attempt to allocate first from their local list, then
// from the global list, and only after those attempts fail will the
// thread attempt to instantiate new monitors. Thread-local free lists
// improve allocation latency, as well as reducing coherency traffic
// on the shared global list.
m = take_from_start_of_om_free_list(self);
if (m != NULL) {
guarantee(m->object() == NULL, "invariant");
m->set_allocation_state(ObjectMonitor::New);
prepend_to_om_in_use_list(self, m);
return m;
}
// 2: try to allocate from the global om_list_globals._free_list
// If we're using thread-local free lists then try
// to reprovision the caller's free list.
// Acquire semantics not needed on this list load since memory
// is already consistent due to the cmpxchg() via
// take_from_start_of_om_free_list() above.
if (Atomic::load(&om_list_globals._free_list) != NULL) {
// Reprovision the thread's om_free_list.
// Use bulk transfers to reduce the allocation rate and heat
// on various locks.
for (int i = self->om_free_provision; --i >= 0;) {
ObjectMonitor* take = take_from_start_of_global_free_list();
if (take == NULL) {
break; // No more are available.
}
guarantee(take->object() == NULL, "invariant");
// We allowed 3 field values to linger during async deflation.
// Clear or restore them as appropriate.
take->set_header(markWord::zero());
// DEFLATER_MARKER is the only non-NULL value we should see here.
take->try_set_owner_from(DEFLATER_MARKER, NULL);
if (take->contentions() < 0) {
// Add back max_jint to restore the contentions field to its
// proper value.
take->add_to_contentions(max_jint);
#ifdef ASSERT
jint l_contentions = take->contentions();
assert(l_contentions >= 0, "must not be negative: l_contentions=%d, contentions=%d",
l_contentions, take->contentions());
#endif
}
take->Recycle();
// Since we're taking from the global free-list, take must be Free.
// om_release() also sets the allocation state to Free because it
// is called from other code paths.
assert(take->is_free(), "invariant");
om_release(self, take, false);
}
self->om_free_provision += 1 + (self->om_free_provision / 2);
if (self->om_free_provision > MAXPRIVATE) self->om_free_provision = MAXPRIVATE;
continue;
}
// 3: allocate a block of new ObjectMonitors
// Both the local and global free lists are empty -- resort to malloc().
// In the current implementation ObjectMonitors are TSM - immortal.
// Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
// each ObjectMonitor to start at the beginning of a cache line,
// so we use align_up().
// A better solution would be to use C++ placement-new.
// BEWARE: As it stands currently, we don't run the ctors!
assert(_BLOCKSIZE > 1, "invariant");
size_t neededsize = sizeof(PaddedObjectMonitor) * _BLOCKSIZE;
PaddedObjectMonitor* temp;
size_t aligned_size = neededsize + (OM_CACHE_LINE_SIZE - 1);
void* real_malloc_addr = NEW_C_HEAP_ARRAY(char, aligned_size, mtInternal);
temp = (PaddedObjectMonitor*)align_up(real_malloc_addr, OM_CACHE_LINE_SIZE);
(void)memset((void *) temp, 0, neededsize);
// Format the block.
// initialize the linked list, each monitor points to its next
// forming the single linked free list, the very first monitor
// will points to next block, which forms the block list.
// The trick of using the 1st element in the block as g_block_list
// linkage should be reconsidered. A better implementation would
// look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
for (int i = 1; i < _BLOCKSIZE; i++) {
temp[i].set_next_om((ObjectMonitor*)&temp[i + 1]);
assert(temp[i].is_free(), "invariant");
}
// terminate the last monitor as the end of list
temp[_BLOCKSIZE - 1].set_next_om((ObjectMonitor*)NULL);
// Element [0] is reserved for global list linkage
temp[0].set_object(CHAINMARKER);
// Consider carving out this thread's current request from the
// block in hand. This avoids some lock traffic and redundant
// list activity.
prepend_block_to_lists(temp);
}
}
// Place "m" on the caller's private per-thread om_free_list.
// In practice there's no need to clamp or limit the number of
// monitors on a thread's om_free_list as the only non-allocation time
// we'll call om_release() is to return a monitor to the free list after
// a CAS attempt failed. This doesn't allow unbounded #s of monitors to
// accumulate on a thread's free list.
//
// Key constraint: all ObjectMonitors on a thread's free list and the global
// free list must have their object field set to null. This prevents the
// scavenger -- deflate_monitor_list_using_JT() -- from reclaiming them
// while we are trying to release them.
void ObjectSynchronizer::om_release(Thread* self, ObjectMonitor* m,
bool from_per_thread_alloc) {
guarantee(m->header().value() == 0, "invariant");
guarantee(m->object() == NULL, "invariant");
NoSafepointVerifier nsv;
if ((m->is_busy() | m->_recursions) != 0) {
stringStream ss;
fatal("freeing in-use monitor: %s, recursions=" INTX_FORMAT,
m->is_busy_to_string(&ss), m->_recursions);
}
m->set_allocation_state(ObjectMonitor::Free);
// _next_om is used for both per-thread in-use and free lists so
// we have to remove 'm' from the in-use list first (as needed).
if (from_per_thread_alloc) {
// Need to remove 'm' from om_in_use_list.
ObjectMonitor* mid = NULL;
ObjectMonitor* next = NULL;
// This list walk can race with another list walker or with async
// deflation so we have to worry about an ObjectMonitor being
// removed from this list while we are walking it.
// Lock the list head to avoid racing with another list walker
// or with async deflation.
if ((mid = get_list_head_locked(&self->om_in_use_list)) == NULL) {
fatal("thread=" INTPTR_FORMAT " in-use list must not be empty.", p2i(self));
}
next = unmarked_next(mid);
if (m == mid) {
// First special case:
// 'm' matches mid, is the list head and is locked. Switch the list
// head to next which unlocks the list head, but leaves the extracted
// mid locked. Release semantics not needed on this "unlock" since
// memory is already consistent due to the get_list_head_locked()
// above.
Atomic::store(&self->om_in_use_list, next);
} else if (m == next) {
// Second special case:
// 'm' matches next after the list head and we already have the list
// head locked so set mid to what we are extracting:
mid = next;
// Lock mid to prevent races with a list walker or an async
// deflater thread that's ahead of us. The locked list head
// prevents races from behind us.
om_lock(mid);
// Update next to what follows mid (if anything):
next = unmarked_next(mid);
// Switch next after the list head to new next which unlocks the
// list head, but leaves the extracted mid locked. Release semantics
// not needed on this "unlock" since memory is already consistent
// due to the get_list_head_locked() above.
self->om_in_use_list->set_next_om(next);
} else {
// We have to search the list to find 'm'.
guarantee(next != NULL, "thread=" INTPTR_FORMAT ": om_in_use_list=" INTPTR_FORMAT
" is too short.", p2i(self), p2i(self->om_in_use_list));
// Our starting anchor is next after the list head which is the
// last ObjectMonitor we checked:
ObjectMonitor* anchor = next;
// Lock anchor to prevent races with a list walker or an async
// deflater thread that's ahead of us. The locked list head
// prevents races from behind us.
om_lock(anchor);
om_unlock(mid); // Unlock the list head now that anchor is locked.
while ((mid = unmarked_next(anchor)) != NULL) {
if (m == mid) {
// We found 'm' on the per-thread in-use list so extract it.
// Update next to what follows mid (if anything):
next = unmarked_next(mid);
// Switch next after the anchor to new next which unlocks the
// anchor, but leaves the extracted mid locked. Release semantics
// not needed on this "unlock" since memory is already consistent
// due to the om_unlock() above before entering the loop or the
// om_unlock() below before looping again.
anchor->set_next_om(next);
break;
} else {
// Lock the next anchor to prevent races with a list walker
// or an async deflater thread that's ahead of us. The locked
// current anchor prevents races from behind us.
om_lock(mid);
// Unlock current anchor now that next anchor is locked:
om_unlock(anchor);
anchor = mid; // Advance to new anchor and try again.
}
}
}
if (mid == NULL) {
// Reached end of the list and didn't find 'm' so:
fatal("thread=" INTPTR_FORMAT " must find m=" INTPTR_FORMAT "on om_in_use_list="
INTPTR_FORMAT, p2i(self), p2i(m), p2i(self->om_in_use_list));
}
// At this point mid is disconnected from the in-use list so
// its lock no longer has any effects on the in-use list.
Atomic::dec(&self->om_in_use_count);
// Unlock mid, but leave the next value for any lagging list
// walkers. It will get cleaned up when mid is prepended to
// the thread's free list:
om_unlock(mid);
}
prepend_to_om_free_list(self, m);
guarantee(m->is_free(), "invariant");
}
// Return ObjectMonitors on a moribund thread's free and in-use
// lists to the appropriate global lists. The ObjectMonitors on the
// per-thread in-use list may still be in use by other threads.
//
// We currently call om_flush() from Threads::remove() before the
// thread has been excised from the thread list and is no longer a
// mutator. In particular, this ensures that the thread's in-use
// monitors are scanned by a GC safepoint, either via Thread::oops_do()
// (before om_flush() is called) or via ObjectSynchronizer::oops_do()
// (after om_flush() is called).
//
// deflate_global_idle_monitors_using_JT() and
// deflate_per_thread_idle_monitors_using_JT() (in another thread) can
// run at the same time as om_flush() so we have to follow a careful
// protocol to prevent list corruption.
void ObjectSynchronizer::om_flush(Thread* self) {
// Process the per-thread in-use list first to be consistent.
int in_use_count = 0;
ObjectMonitor* in_use_list = NULL;
ObjectMonitor* in_use_tail = NULL;
NoSafepointVerifier nsv;
// This function can race with a list walker or with an async
// deflater thread so we lock the list head to prevent confusion.
// An async deflater thread checks to see if the target thread
// is exiting, but if it has made it past that check before we
// started exiting, then it is racing to get to the in-use list.
if ((in_use_list = get_list_head_locked(&self->om_in_use_list)) != NULL) {
// At this point, we have locked the in-use list head so a racing
// thread cannot come in after us. However, a racing thread could
// be ahead of us; we'll detect that and delay to let it finish.
//
// The thread is going away, however the ObjectMonitors on the
// om_in_use_list may still be in-use by other threads. Link
// them to in_use_tail, which will be linked into the global
// in-use list (om_list_globals._in_use_list) below.
//
// Account for the in-use list head before the loop since it is
// already locked (by this thread):
in_use_tail = in_use_list;
in_use_count++;
for (ObjectMonitor* cur_om = unmarked_next(in_use_list); cur_om != NULL;) {
if (is_locked(cur_om)) {
// cur_om is locked so there must be a racing walker or async
// deflater thread ahead of us so we'll give it a chance to finish.
while (is_locked(cur_om)) {
os::naked_short_sleep(1);
}
// Refetch the possibly changed next field and try again.
cur_om = unmarked_next(in_use_tail);
continue;
}
if (cur_om->object() == NULL) {
// cur_om was deflated and the object ref was cleared while it
// was locked. We happened to see it just after it was unlocked
// (and added to the free list). Refetch the possibly changed
// next field and try again.
cur_om = unmarked_next(in_use_tail);
continue;
}
in_use_tail = cur_om;
in_use_count++;
cur_om = unmarked_next(cur_om);
}
guarantee(in_use_tail != NULL, "invariant");
#ifdef ASSERT
int l_om_in_use_count = Atomic::load(&self->om_in_use_count);
assert(l_om_in_use_count == in_use_count, "in-use counts don't match: "
"l_om_in_use_count=%d, in_use_count=%d", l_om_in_use_count, in_use_count);
#endif
Atomic::store(&self->om_in_use_count, 0);
OrderAccess::storestore(); // Make sure counter update is seen first.
// Clear the in-use list head (which also unlocks it):
Atomic::store(&self->om_in_use_list, (ObjectMonitor*)NULL);
om_unlock(in_use_list);
}
int free_count = 0;
ObjectMonitor* free_list = NULL;
ObjectMonitor* free_tail = NULL;
// This function can race with a list walker thread so we lock the
// list head to prevent confusion.
if ((free_list = get_list_head_locked(&self->om_free_list)) != NULL) {
// At this point, we have locked the free list head so a racing
// thread cannot come in after us. However, a racing thread could
// be ahead of us; we'll detect that and delay to let it finish.
//
// The thread is going away. Set 'free_tail' to the last per-thread free
// monitor which will be linked to om_list_globals._free_list below.
//
// Account for the free list head before the loop since it is
// already locked (by this thread):
free_tail = free_list;
free_count++;
for (ObjectMonitor* s = unmarked_next(free_list); s != NULL; s = unmarked_next(s)) {
if (is_locked(s)) {
// s is locked so there must be a racing walker thread ahead
// of us so we'll give it a chance to finish.
while (is_locked(s)) {
os::naked_short_sleep(1);
}
}
free_tail = s;
free_count++;
guarantee(s->object() == NULL, "invariant");
if (s->is_busy()) {
stringStream ss;
fatal("must be !is_busy: %s", s->is_busy_to_string(&ss));
}
}
guarantee(free_tail != NULL, "invariant");
#ifdef ASSERT
int l_om_free_count = Atomic::load(&self->om_free_count);
assert(l_om_free_count == free_count, "free counts don't match: "
"l_om_free_count=%d, free_count=%d", l_om_free_count, free_count);
#endif
Atomic::store(&self->om_free_count, 0);
OrderAccess::storestore(); // Make sure counter update is seen first.
Atomic::store(&self->om_free_list, (ObjectMonitor*)NULL);
om_unlock(free_list);
}
if (free_tail != NULL) {
prepend_list_to_global_free_list(free_list, free_tail, free_count);
}
if (in_use_tail != NULL) {
prepend_list_to_global_in_use_list(in_use_list, in_use_tail, in_use_count);
}
LogStreamHandle(Debug, monitorinflation) lsh_debug;
LogStreamHandle(Info, monitorinflation) lsh_info;
LogStream* ls = NULL;
if (log_is_enabled(Debug, monitorinflation)) {
ls = &lsh_debug;
} else if ((free_count != 0 || in_use_count != 0) &&
log_is_enabled(Info, monitorinflation)) {
ls = &lsh_info;
}
if (ls != NULL) {
ls->print_cr("om_flush: jt=" INTPTR_FORMAT ", free_count=%d"
", in_use_count=%d" ", om_free_provision=%d",
p2i(self), free_count, in_use_count, self->om_free_provision);
}
}
static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
const oop obj,
ObjectSynchronizer::InflateCause cause) {
assert(event != NULL, "invariant");
assert(event->should_commit(), "invariant");
event->set_monitorClass(obj->klass());
event->set_address((uintptr_t)(void*)obj);
event->set_cause((u1)cause);
event->commit();
}
// Fast path code shared by multiple functions
void ObjectSynchronizer::inflate_helper(oop obj) {
markWord mark = obj->mark();
if (mark.has_monitor()) {
ObjectMonitor* monitor = mark.monitor();
assert(ObjectSynchronizer::verify_objmon_isinpool(monitor), "monitor=" INTPTR_FORMAT " is invalid", p2i(monitor));
markWord dmw = monitor->header();
assert(dmw.is_neutral(), "sanity check: header=" INTPTR_FORMAT, dmw.value());
return;
}
(void)inflate(Thread::current(), obj, inflate_cause_vm_internal);
}
ObjectMonitor* ObjectSynchronizer::inflate(Thread* self, oop object,
const InflateCause cause) {
EventJavaMonitorInflate event;
for (;;) {
const markWord mark = object->mark();
assert(!mark.has_bias_pattern(), "invariant");
// The mark can be in one of the following states:
// * Inflated - just return
// * Stack-locked - coerce it to inflated
// * INFLATING - busy wait for conversion to complete
// * Neutral - aggressively inflate the object.
// * BIASED - Illegal. We should never see this
// CASE: inflated
if (mark.has_monitor()) {
ObjectMonitor* inf = mark.monitor();
markWord dmw = inf->header();
assert(dmw.is_neutral(), "invariant: header=" INTPTR_FORMAT, dmw.value());
assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
return inf;
}
// CASE: inflation in progress - inflating over a stack-lock.
// Some other thread is converting from stack-locked to inflated.
// Only that thread can complete inflation -- other threads must wait.
// The INFLATING value is transient.
// Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
// We could always eliminate polling by parking the thread on some auxiliary list.
if (mark == markWord::INFLATING()) {
read_stable_mark(object);
continue;
}
// CASE: stack-locked
// Could be stack-locked either by this thread or by some other thread.
//
// Note that we allocate the objectmonitor speculatively, _before_ attempting
// to install INFLATING into the mark word. We originally installed INFLATING,
// allocated the objectmonitor, and then finally STed the address of the
// objectmonitor into the mark. This was correct, but artificially lengthened
// the interval in which INFLATED appeared in the mark, thus increasing
// the odds of inflation contention.
//
// We now use per-thread private objectmonitor free lists.
// These list are reprovisioned from the global free list outside the
// critical INFLATING...ST interval. A thread can transfer
// multiple objectmonitors en-mass from the global free list to its local free list.
// This reduces coherency traffic and lock contention on the global free list.
// Using such local free lists, it doesn't matter if the om_alloc() call appears
// before or after the CAS(INFLATING) operation.
// See the comments in om_alloc().
LogStreamHandle(Trace, monitorinflation) lsh;
if (mark.has_locker()) {
ObjectMonitor* m = om_alloc(self);
// Optimistically prepare the objectmonitor - anticipate successful CAS
// We do this before the CAS in order to minimize the length of time
// in which INFLATING appears in the mark.
m->Recycle();
m->_Responsible = NULL;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
markWord cmp = object->cas_set_mark(markWord::INFLATING(), mark);
if (cmp != mark) {
// om_release() will reset the allocation state from New to Free.
om_release(self, m, true);
continue; // Interference -- just retry
}
// We've successfully installed INFLATING (0) into the mark-word.
// This is the only case where 0 will appear in a mark-word.
// Only the singular thread that successfully swings the mark-word
// to 0 can perform (or more precisely, complete) inflation.
//
// Why do we CAS a 0 into the mark-word instead of just CASing the
// mark-word from the stack-locked value directly to the new inflated state?
// Consider what happens when a thread unlocks a stack-locked object.
// It attempts to use CAS to swing the displaced header value from the
// on-stack BasicLock back into the object header. Recall also that the
// header value (hash code, etc) can reside in (a) the object header, or
// (b) a displaced header associated with the stack-lock, or (c) a displaced
// header in an ObjectMonitor. The inflate() routine must copy the header
// value from the BasicLock on the owner's stack to the ObjectMonitor, all
// the while preserving the hashCode stability invariants. If the owner
// decides to release the lock while the value is 0, the unlock will fail
// and control will eventually pass from slow_exit() to inflate. The owner
// will then spin, waiting for the 0 value to disappear. Put another way,
// the 0 causes the owner to stall if the owner happens to try to
// drop the lock (restoring the header from the BasicLock to the object)
// while inflation is in-progress. This protocol avoids races that might
// would otherwise permit hashCode values to change or "flicker" for an object.
// Critically, while object->mark is 0 mark.displaced_mark_helper() is stable.
// 0 serves as a "BUSY" inflate-in-progress indicator.
// fetch the displaced mark from the owner's stack.
// The owner can't die or unwind past the lock while our INFLATING
// object is in the mark. Furthermore the owner can't complete
// an unlock on the object, either.
markWord dmw = mark.displaced_mark_helper();
// Catch if the object's header is not neutral (not locked and
// not marked is what we care about here).
assert(dmw.is_neutral(), "invariant: header=" INTPTR_FORMAT, dmw.value());
// Setup monitor fields to proper values -- prepare the monitor
m->set_header(dmw);
// Optimization: if the mark.locker stack address is associated
// with this thread we could simply set m->_owner = self.
// Note that a thread can inflate an object
// that it has stack-locked -- as might happen in wait() -- directly
// with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
m->set_owner_from(NULL, DEFLATER_MARKER, mark.locker());
m->set_object(object);
// TODO-FIXME: assert BasicLock->dhw != 0.
// Must preserve store ordering. The monitor state must
// be stable at the time of publishing the monitor address.
guarantee(object->mark() == markWord::INFLATING(), "invariant");
// Release semantics so that above set_object() is seen first.
object->release_set_mark(markWord::encode(m));
// Once ObjectMonitor is configured and the object is associated
// with the ObjectMonitor, it is safe to allow async deflation:
assert(m->is_new(), "freshly allocated monitor must be new");
// Release semantics needed to keep allocation_state from floating up.
m->release_set_allocation_state(ObjectMonitor::Old);
// Hopefully the performance counters are allocated on distinct cache lines
// to avoid false sharing on MP systems ...
OM_PERFDATA_OP(Inflations, inc());
if (log_is_enabled(Trace, monitorinflation)) {
ResourceMark rm(self);
lsh.print_cr("inflate(has_locker): object=" INTPTR_FORMAT ", mark="
INTPTR_FORMAT ", type='%s'", p2i(object),
object->mark().value(), object->klass()->external_name());
}
if (event.should_commit()) {
post_monitor_inflate_event(&event, object, cause);
}
return m;
}
// CASE: neutral
// TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
// If we know we're inflating for entry it's better to inflate by swinging a
// pre-locked ObjectMonitor pointer into the object header. A successful
// CAS inflates the object *and* confers ownership to the inflating thread.
// In the current implementation we use a 2-step mechanism where we CAS()
// to inflate and then CAS() again to try to swing _owner from NULL to self.
// An inflateTry() method that we could call from enter() would be useful.
// Catch if the object's header is not neutral (not locked and
// not marked is what we care about here).
assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value());
ObjectMonitor* m = om_alloc(self);
// prepare m for installation - set monitor to initial state
m->Recycle();
m->set_header(mark);
// DEFLATER_MARKER is the only non-NULL value we should see here.
m->try_set_owner_from(DEFLATER_MARKER, NULL);
m->set_object(object);
m->_Responsible = NULL;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
if (object->cas_set_mark(markWord::encode(m), mark) != mark) {
m->set_header(markWord::zero());
m->set_object(NULL);
m->Recycle();
// om_release() will reset the allocation state from New to Free.
om_release(self, m, true);
m = NULL;
continue;
// interference - the markword changed - just retry.
// The state-transitions are one-way, so there's no chance of
// live-lock -- "Inflated" is an absorbing state.
}
// Once the ObjectMonitor is configured and object is associated
// with the ObjectMonitor, it is safe to allow async deflation:
assert(m->is_new(), "freshly allocated monitor must be new");
// Release semantics are not needed to keep allocation_state from
// floating up since cas_set_mark() takes care of it.
m->set_allocation_state(ObjectMonitor::Old);
// Hopefully the performance counters are allocated on distinct
// cache lines to avoid false sharing on MP systems ...
OM_PERFDATA_OP(Inflations, inc());
if (log_is_enabled(Trace, monitorinflation)) {
ResourceMark rm(self);
lsh.print_cr("inflate(neutral): object=" INTPTR_FORMAT ", mark="
INTPTR_FORMAT ", type='%s'", p2i(object),
object->mark().value(), object->klass()->external_name());
}
if (event.should_commit()) {
post_monitor_inflate_event(&event, object, cause);
}
return m;
}
}
// An async deflation request is registered with the ServiceThread
// and it is notified.
void ObjectSynchronizer::do_safepoint_work() {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
log_debug(monitorinflation)("requesting async deflation of idle monitors.");
// Request deflation of idle monitors by the ServiceThread:
set_is_async_deflation_requested(true);
MonitorLocker ml(Service_lock, Mutex::_no_safepoint_check_flag);
ml.notify_all();
if (log_is_enabled(Debug, monitorinflation)) {
// exit_globals()'s call to audit_and_print_stats() is done
// at the Info level and not at a safepoint.
ObjectSynchronizer::audit_and_print_stats(false /* on_exit */);
}
}
// Deflate the specified ObjectMonitor if not in-use using a JavaThread.
// Returns true if it was deflated and false otherwise.
//
// The async deflation protocol sets owner to DEFLATER_MARKER and
// makes contentions negative as signals to contending threads that
// an async deflation is in progress. There are a number of checks
// as part of the protocol to make sure that the calling thread has
// not lost the race to a contending thread.
//
// The ObjectMonitor has been successfully async deflated when:
// (contentions < 0)
// Contending threads that see that condition know to retry their operation.
//
bool ObjectSynchronizer::deflate_monitor_using_JT(ObjectMonitor* mid,
ObjectMonitor** free_head_p,
ObjectMonitor** free_tail_p) {
assert(Thread::current()->is_Java_thread(), "precondition");
// A newly allocated ObjectMonitor should not be seen here so we
// avoid an endless inflate/deflate cycle.
assert(mid->is_old(), "must be old: allocation_state=%d",
(int) mid->allocation_state());
if (mid->is_busy()) {
// Easy checks are first - the ObjectMonitor is busy so no deflation.
return false;
}
// Set a NULL owner to DEFLATER_MARKER to force any contending thread
// through the slow path. This is just the first part of the async
// deflation dance.
if (mid->try_set_owner_from(NULL, DEFLATER_MARKER) != NULL) {
// The owner field is no longer NULL so we lost the race since the
// ObjectMonitor is now busy.
return false;
}
if (mid->contentions() > 0 || mid->_waiters != 0) {
// Another thread has raced to enter the ObjectMonitor after
// mid->is_busy() above or has already entered and waited on
// it which makes it busy so no deflation. Restore owner to
// NULL if it is still DEFLATER_MARKER.
if (mid->try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
// Deferred decrement for the JT EnterI() that cancelled the async deflation.
mid->add_to_contentions(-1);
}
return false;
}
// Make a zero contentions field negative to force any contending threads
// to retry. This is the second part of the async deflation dance.
if (Atomic::cmpxchg(&mid->_contentions, (jint)0, -max_jint) != 0) {
// Contentions was no longer 0 so we lost the race since the
// ObjectMonitor is now busy. Restore owner to NULL if it is
// still DEFLATER_MARKER:
if (mid->try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
// Deferred decrement for the JT EnterI() that cancelled the async deflation.
mid->add_to_contentions(-1);
}
return false;
}
// Sanity checks for the races:
guarantee(mid->owner_is_DEFLATER_MARKER(), "must be deflater marker");
guarantee(mid->contentions() < 0, "must be negative: contentions=%d",
mid->contentions());
guarantee(mid->_waiters == 0, "must be 0: waiters=%d", mid->_waiters);
guarantee(mid->_cxq == NULL, "must be no contending threads: cxq="
INTPTR_FORMAT, p2i(mid->_cxq));
guarantee(mid->_EntryList == NULL,
"must be no entering threads: EntryList=" INTPTR_FORMAT,
p2i(mid->_EntryList));
const oop obj = (oop) mid->object();
if (log_is_enabled(Trace, monitorinflation)) {
ResourceMark rm;
log_trace(monitorinflation)("deflate_monitor_using_JT: "
"object=" INTPTR_FORMAT ", mark="
INTPTR_FORMAT ", type='%s'",
p2i(obj), obj->mark().value(),
obj->klass()->external_name());
}
// Install the old mark word if nobody else has already done it.
mid->install_displaced_markword_in_object(obj);
mid->clear_common();
assert(mid->object() == NULL, "must be NULL: object=" INTPTR_FORMAT,
p2i(mid->object()));
assert(mid->is_free(), "must be free: allocation_state=%d",
(int)mid->allocation_state());
// Move the deflated ObjectMonitor to the working free list
// defined by free_head_p and free_tail_p.
if (*free_head_p == NULL) {
// First one on the list.
*free_head_p = mid;
}
if (*free_tail_p != NULL) {
// We append to the list so the caller can use mid->_next_om
// to fix the linkages in its context.
ObjectMonitor* prevtail = *free_tail_p;
// prevtail should have been cleaned up by the caller:
#ifdef ASSERT
ObjectMonitor* l_next_om = unmarked_next(prevtail);
assert(l_next_om == NULL, "must be NULL: _next_om=" INTPTR_FORMAT, p2i(l_next_om));
#endif
om_lock(prevtail);
prevtail->set_next_om(mid); // prevtail now points to mid (and is unlocked)
}
*free_tail_p = mid;
// At this point, mid->_next_om still refers to its current
// value and another ObjectMonitor's _next_om field still
// refers to this ObjectMonitor. Those linkages have to be
// cleaned up by the caller who has the complete context.
// We leave owner == DEFLATER_MARKER and contentions < 0
// to force any racing threads to retry.
return true; // Success, ObjectMonitor has been deflated.
}
// Walk a given ObjectMonitor list and deflate idle ObjectMonitors using
// a JavaThread. Returns the number of deflated ObjectMonitors. The given
// list could be a per-thread in-use list or the global in-use list.
// If a safepoint has started, then we save state via saved_mid_in_use_p
// and return to the caller to honor the safepoint.
//
int ObjectSynchronizer::deflate_monitor_list_using_JT(ObjectMonitor** list_p,
int* count_p,
ObjectMonitor** free_head_p,
ObjectMonitor** free_tail_p,
ObjectMonitor** saved_mid_in_use_p) {
JavaThread* self = JavaThread::current();
ObjectMonitor* cur_mid_in_use = NULL;
ObjectMonitor* mid = NULL;
ObjectMonitor* next = NULL;
ObjectMonitor* next_next = NULL;
int deflated_count = 0;
NoSafepointVerifier nsv;
// We use the more complicated lock-cur_mid_in_use-and-mid-as-we-go
// protocol because om_release() can do list deletions in parallel;
// this also prevents races with a list walker thread. We also
// lock-next-next-as-we-go to prevent an om_flush() that is behind
// this thread from passing us.
if (*saved_mid_in_use_p == NULL) {
// No saved state so start at the beginning.
// Lock the list head so we can possibly deflate it:
if ((mid = get_list_head_locked(list_p)) == NULL) {
return 0; // The list is empty so nothing to deflate.
}
next = unmarked_next(mid);
} else {
// We're restarting after a safepoint so restore the necessary state
// before we resume.
cur_mid_in_use = *saved_mid_in_use_p;
// Lock cur_mid_in_use so we can possibly update its
// next field to extract a deflated ObjectMonitor.
om_lock(cur_mid_in_use);
mid = unmarked_next(cur_mid_in_use);
if (mid == NULL) {
om_unlock(cur_mid_in_use);
*saved_mid_in_use_p = NULL;
return 0; // The remainder is empty so nothing more to deflate.
}
// Lock mid so we can possibly deflate it:
om_lock(mid);
next = unmarked_next(mid);
}
while (true) {
// The current mid is locked at this point. If we have a
// cur_mid_in_use, then it is also locked at this point.
if (next != NULL) {
// We lock next so that an om_flush() thread that is behind us
// cannot pass us when we unlock the current mid.
om_lock(next);
next_next = unmarked_next(next);
}
// Only try to deflate if there is an associated Java object and if
// mid is old (is not newly allocated and is not newly freed).
if (mid->object() != NULL && mid->is_old() &&
deflate_monitor_using_JT(mid, free_head_p, free_tail_p)) {
// Deflation succeeded and already updated free_head_p and
// free_tail_p as needed. Finish the move to the local free list
// by unlinking mid from the global or per-thread in-use list.
if (cur_mid_in_use == NULL) {
// mid is the list head and it is locked. Switch the list head
// to next which is also locked (if not NULL) and also leave
// mid locked. Release semantics needed since not all code paths
// in deflate_monitor_using_JT() ensure memory consistency.
Atomic::release_store(list_p, next);
} else {
ObjectMonitor* locked_next = mark_om_ptr(next);
// mid and cur_mid_in_use are locked. Switch cur_mid_in_use's
// next field to locked_next and also leave mid locked.
// Release semantics needed since not all code paths in
// deflate_monitor_using_JT() ensure memory consistency.
cur_mid_in_use->release_set_next_om(locked_next);
}
// At this point mid is disconnected from the in-use list so
// its lock longer has any effects on in-use list.
deflated_count++;
Atomic::dec(count_p);
// mid is current tail in the free_head_p list so NULL terminate
// it (which also unlocks it). No release semantics needed since
// Atomic::dec() already provides it.
mid->set_next_om(NULL);
// All the list management is done so move on to the next one:
mid = next; // mid keeps non-NULL next's locked state
next = next_next;
} else {
// mid is considered in-use if it does not have an associated
// Java object or mid is not old or deflation did not succeed.
// A mid->is_new() node can be seen here when it is freshly
// returned by om_alloc() (and skips the deflation code path).
// A mid->is_old() node can be seen here when deflation failed.
// A mid->is_free() node can be seen here when a fresh node from
// om_alloc() is released by om_release() due to losing the race
// in inflate().
// All the list management is done so move on to the next one:
if (cur_mid_in_use != NULL) {
om_unlock(cur_mid_in_use);
}
// The next cur_mid_in_use keeps mid's lock state so
// that it is stable for a possible next field change. It
// cannot be modified by om_release() while it is locked.
cur_mid_in_use = mid;
mid = next; // mid keeps non-NULL next's locked state
next = next_next;
if (SafepointMechanism::should_block(self) &&
// Acquire semantics are not needed on this list load since
// it is not dependent on the following load which does have
// acquire semantics.
cur_mid_in_use != Atomic::load(list_p) && cur_mid_in_use->is_old()) {
// If a safepoint has started and cur_mid_in_use is not the list
// head and is old, then it is safe to use as saved state. Return
// to the caller before blocking.
*saved_mid_in_use_p = cur_mid_in_use;
om_unlock(cur_mid_in_use);
if (mid != NULL) {
om_unlock(mid);
}
return deflated_count;
}
}
if (mid == NULL) {
if (cur_mid_in_use != NULL) {
om_unlock(cur_mid_in_use);
}
break; // Reached end of the list so nothing more to deflate.
}
// The current mid's next field is locked at this point. If we have
// a cur_mid_in_use, then it is also locked at this point.
}
// We finished the list without a safepoint starting so there's
// no need to save state.
*saved_mid_in_use_p = NULL;
return deflated_count;
}
class HandshakeForDeflation : public HandshakeClosure {
public:
HandshakeForDeflation() : HandshakeClosure("HandshakeForDeflation") {}
void do_thread(Thread* thread) {
log_trace(monitorinflation)("HandshakeForDeflation::do_thread: thread="
INTPTR_FORMAT, p2i(thread));
}
};
void ObjectSynchronizer::deflate_idle_monitors_using_JT() {
// Deflate any global idle monitors.
deflate_global_idle_monitors_using_JT();
int count = 0;
for (JavaThreadIteratorWithHandle jtiwh; JavaThread *jt = jtiwh.next(); ) {
if (Atomic::load(&jt->om_in_use_count) > 0 && !jt->is_exiting()) {
// This JavaThread is using ObjectMonitors so deflate any that
// are idle unless this JavaThread is exiting; do not race with
// ObjectSynchronizer::om_flush().
deflate_per_thread_idle_monitors_using_JT(jt);
count++;
}
}
if (count > 0) {
log_debug(monitorinflation)("did async deflation of idle monitors for %d thread(s).", count);
}
log_info(monitorinflation)("async global_population=%d, global_in_use_count=%d, "
"global_free_count=%d, global_wait_count=%d",
Atomic::load(&om_list_globals._population),
Atomic::load(&om_list_globals._in_use_count),
Atomic::load(&om_list_globals._free_count),
Atomic::load(&om_list_globals._wait_count));
// The ServiceThread's async deflation request has been processed.
_last_async_deflation_time_ns = os::javaTimeNanos();
set_is_async_deflation_requested(false);
if (Atomic::load(&om_list_globals._wait_count) > 0) {
// There are deflated ObjectMonitors waiting for a handshake
// (or a safepoint) for safety.
ObjectMonitor* list = Atomic::load(&om_list_globals._wait_list);
assert(list != NULL, "om_list_globals._wait_list must not be NULL");
int count = Atomic::load(&om_list_globals._wait_count);
Atomic::store(&om_list_globals._wait_count, 0);
OrderAccess::storestore(); // Make sure counter update is seen first.
Atomic::store(&om_list_globals._wait_list, (ObjectMonitor*)NULL);
// Find the tail for prepend_list_to_common(). No need to mark
// ObjectMonitors for this list walk since only the deflater
// thread manages the wait list.
#ifdef ASSERT
int l_count = 0;
#endif
ObjectMonitor* tail = NULL;
for (ObjectMonitor* n = list; n != NULL; n = unmarked_next(n)) {
tail = n;
#ifdef ASSERT
l_count++;
#endif
}
assert(count == l_count, "count=%d != l_count=%d", count, l_count);
// Will execute a safepoint if !ThreadLocalHandshakes:
HandshakeForDeflation hfd_hc;
Handshake::execute(&hfd_hc);
prepend_list_to_common(list, tail, count, &om_list_globals._free_list,
&om_list_globals._free_count);
log_info(monitorinflation)("moved %d idle monitors from global waiting list to global free list", count);
}
}
// Deflate global idle ObjectMonitors using a JavaThread.
//
void ObjectSynchronizer::deflate_global_idle_monitors_using_JT() {
assert(Thread::current()->is_Java_thread(), "precondition");
JavaThread* self = JavaThread::current();
deflate_common_idle_monitors_using_JT(true /* is_global */, self);
}
// Deflate the specified JavaThread's idle ObjectMonitors using a JavaThread.
//
void ObjectSynchronizer::deflate_per_thread_idle_monitors_using_JT(JavaThread* target) {
assert(Thread::current()->is_Java_thread(), "precondition");
deflate_common_idle_monitors_using_JT(false /* !is_global */, target);
}
// Deflate global or per-thread idle ObjectMonitors using a JavaThread.
//
void ObjectSynchronizer::deflate_common_idle_monitors_using_JT(bool is_global, JavaThread* target) {
JavaThread* self = JavaThread::current();
int deflated_count = 0;
ObjectMonitor* free_head_p = NULL; // Local SLL of scavenged ObjectMonitors
ObjectMonitor* free_tail_p = NULL;
ObjectMonitor* saved_mid_in_use_p = NULL;
elapsedTimer timer;
if (log_is_enabled(Info, monitorinflation)) {
timer.start();
}
if (is_global) {
OM_PERFDATA_OP(MonExtant, set_value(Atomic::load(&om_list_globals._in_use_count)));
} else {
OM_PERFDATA_OP(MonExtant, inc(Atomic::load(&target->om_in_use_count)));
}
do {
int local_deflated_count;
if (is_global) {
local_deflated_count =
deflate_monitor_list_using_JT(&om_list_globals._in_use_list,
&om_list_globals._in_use_count,
&free_head_p, &free_tail_p,
&saved_mid_in_use_p);
} else {
local_deflated_count =
deflate_monitor_list_using_JT(&target->om_in_use_list,
&target->om_in_use_count, &free_head_p,
&free_tail_p, &saved_mid_in_use_p);
}
deflated_count += local_deflated_count;
if (free_head_p != NULL) {
// Move the deflated ObjectMonitors to the global free list.
guarantee(free_tail_p != NULL && local_deflated_count > 0, "free_tail_p=" INTPTR_FORMAT ", local_deflated_count=%d", p2i(free_tail_p), local_deflated_count);
// Note: The target thread can be doing an om_alloc() that
// is trying to prepend an ObjectMonitor on its in-use list
// at the same time that we have deflated the current in-use
// list head and put it on the local free list. prepend_to_common()
// will detect the race and retry which avoids list corruption,
// but the next field in free_tail_p can flicker to marked
// and then unmarked while prepend_to_common() is sorting it
// all out.
#ifdef ASSERT
ObjectMonitor* l_next_om = unmarked_next(free_tail_p);
assert(l_next_om == NULL, "must be NULL: _next_om=" INTPTR_FORMAT, p2i(l_next_om));
#endif
prepend_list_to_global_wait_list(free_head_p, free_tail_p, local_deflated_count);
OM_PERFDATA_OP(Deflations, inc(local_deflated_count));
}
if (saved_mid_in_use_p != NULL) {
// deflate_monitor_list_using_JT() detected a safepoint starting.
timer.stop();
{
if (is_global) {
log_debug(monitorinflation)("pausing deflation of global idle monitors for a safepoint.");
} else {
log_debug(monitorinflation)("jt=" INTPTR_FORMAT ": pausing deflation of per-thread idle monitors for a safepoint.", p2i(target));
}
assert(SafepointMechanism::should_block(self), "sanity check");
ThreadBlockInVM blocker(self);
}
// Prepare for another loop after the safepoint.
free_head_p = NULL;
free_tail_p = NULL;
if (log_is_enabled(Info, monitorinflation)) {
timer.start();
}
}
} while (saved_mid_in_use_p != NULL);
timer.stop();
LogStreamHandle(Debug, monitorinflation) lsh_debug;
LogStreamHandle(Info, monitorinflation) lsh_info;
LogStream* ls = NULL;
if (log_is_enabled(Debug, monitorinflation)) {
ls = &lsh_debug;
} else if (deflated_count != 0 && log_is_enabled(Info, monitorinflation)) {
ls = &lsh_info;
}
if (ls != NULL) {
if (is_global) {
ls->print_cr("async-deflating global idle monitors, %3.7f secs, %d monitors", timer.seconds(), deflated_count);
} else {
ls->print_cr("jt=" INTPTR_FORMAT ": async-deflating per-thread idle monitors, %3.7f secs, %d monitors", p2i(target), timer.seconds(), deflated_count);
}
}
}
// Monitor cleanup on JavaThread::exit
// Iterate through monitor cache and attempt to release thread's monitors
// Gives up on a particular monitor if an exception occurs, but continues
// the overall iteration, swallowing the exception.
class ReleaseJavaMonitorsClosure: public MonitorClosure {
private:
TRAPS;
public:
ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
void do_monitor(ObjectMonitor* mid) {
if (mid->owner() == THREAD) {
(void)mid->complete_exit(CHECK);
}
}
};
// Release all inflated monitors owned by THREAD. Lightweight monitors are
// ignored. This is meant to be called during JNI thread detach which assumes
// all remaining monitors are heavyweight. All exceptions are swallowed.
// Scanning the extant monitor list can be time consuming.
// A simple optimization is to add a per-thread flag that indicates a thread
// called jni_monitorenter() during its lifetime.
//
// Instead of NoSafepointVerifier it might be cheaper to
// use an idiom of the form:
// auto int tmp = SafepointSynchronize::_safepoint_counter ;
// <code that must not run at safepoint>
// guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
// Since the tests are extremely cheap we could leave them enabled
// for normal product builds.
void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
assert(THREAD == JavaThread::current(), "must be current Java thread");
NoSafepointVerifier nsv;
ReleaseJavaMonitorsClosure rjmc(THREAD);
ObjectSynchronizer::monitors_iterate(&rjmc);
THREAD->clear_pending_exception();
}
const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
switch (cause) {
case inflate_cause_vm_internal: return "VM Internal";
case inflate_cause_monitor_enter: return "Monitor Enter";
case inflate_cause_wait: return "Monitor Wait";
case inflate_cause_notify: return "Monitor Notify";
case inflate_cause_hash_code: return "Monitor Hash Code";
case inflate_cause_jni_enter: return "JNI Monitor Enter";
case inflate_cause_jni_exit: return "JNI Monitor Exit";
default:
ShouldNotReachHere();
}
return "Unknown";
}
//------------------------------------------------------------------------------
// Debugging code
u_char* ObjectSynchronizer::get_gvars_addr() {
return (u_char*)&GVars;
}
u_char* ObjectSynchronizer::get_gvars_hc_sequence_addr() {
return (u_char*)&GVars.hc_sequence;
}
size_t ObjectSynchronizer::get_gvars_size() {
return sizeof(SharedGlobals);
}
u_char* ObjectSynchronizer::get_gvars_stw_random_addr() {
return (u_char*)&GVars.stw_random;
}
// This function can be called at a safepoint or it can be called when
// we are trying to exit the VM. When we are trying to exit the VM, the
// list walker functions can run in parallel with the other list
// operations so spin-locking is used for safety.
//
// Calls to this function can be added in various places as a debugging
// aid; pass 'true' for the 'on_exit' parameter to have in-use monitor
// details logged at the Info level and 'false' for the 'on_exit'
// parameter to have in-use monitor details logged at the Trace level.
//
void ObjectSynchronizer::audit_and_print_stats(bool on_exit) {
assert(on_exit || SafepointSynchronize::is_at_safepoint(), "invariant");
LogStreamHandle(Debug, monitorinflation) lsh_debug;
LogStreamHandle(Info, monitorinflation) lsh_info;
LogStreamHandle(Trace, monitorinflation) lsh_trace;
LogStream* ls = NULL;
if (log_is_enabled(Trace, monitorinflation)) {
ls = &lsh_trace;
} else if (log_is_enabled(Debug, monitorinflation)) {
ls = &lsh_debug;
} else if (log_is_enabled(Info, monitorinflation)) {
ls = &lsh_info;
}
assert(ls != NULL, "sanity check");
// Log counts for the global and per-thread monitor lists:
int chk_om_population = log_monitor_list_counts(ls);
int error_cnt = 0;
ls->print_cr("Checking global lists:");
// Check om_list_globals._population:
if (Atomic::load(&om_list_globals._population) == chk_om_population) {
ls->print_cr("global_population=%d equals chk_om_population=%d",
Atomic::load(&om_list_globals._population), chk_om_population);
} else {
// With fine grained locks on the monitor lists, it is possible for
// log_monitor_list_counts() to return a value that doesn't match
// om_list_globals._population. So far a higher value has been
// seen in testing so something is being double counted by
// log_monitor_list_counts().
ls->print_cr("WARNING: global_population=%d is not equal to "
"chk_om_population=%d",
Atomic::load(&om_list_globals._population), chk_om_population);
}
// Check om_list_globals._in_use_list and om_list_globals._in_use_count:
chk_global_in_use_list_and_count(ls, &error_cnt);
// Check om_list_globals._free_list and om_list_globals._free_count:
chk_global_free_list_and_count(ls, &error_cnt);
// Check om_list_globals._wait_list and om_list_globals._wait_count:
chk_global_wait_list_and_count(ls, &error_cnt);
ls->print_cr("Checking per-thread lists:");
for (JavaThreadIteratorWithHandle jtiwh; JavaThread *jt = jtiwh.next(); ) {
// Check om_in_use_list and om_in_use_count:
chk_per_thread_in_use_list_and_count(jt, ls, &error_cnt);
// Check om_free_list and om_free_count:
chk_per_thread_free_list_and_count(jt, ls, &error_cnt);
}
if (error_cnt == 0) {
ls->print_cr("No errors found in monitor list checks.");
} else {
log_error(monitorinflation)("found monitor list errors: error_cnt=%d", error_cnt);
}
if ((on_exit && log_is_enabled(Info, monitorinflation)) ||
(!on_exit && log_is_enabled(Trace, monitorinflation))) {
// When exiting this log output is at the Info level. When called
// at a safepoint, this log output is at the Trace level since
// there can be a lot of it.
log_in_use_monitor_details(ls);
}
ls->flush();
guarantee(error_cnt == 0, "ERROR: found monitor list errors: error_cnt=%d", error_cnt);
}
// Check a free monitor entry; log any errors.
void ObjectSynchronizer::chk_free_entry(JavaThread* jt, ObjectMonitor* n,
outputStream * out, int *error_cnt_p) {
stringStream ss;
if (n->is_busy()) {
if (jt != NULL) {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ", monitor=" INTPTR_FORMAT
": free per-thread monitor must not be busy: %s", p2i(jt),
p2i(n), n->is_busy_to_string(&ss));
} else {
out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": free global monitor "
"must not be busy: %s", p2i(n), n->is_busy_to_string(&ss));
}
*error_cnt_p = *error_cnt_p + 1;
}
if (n->header().value() != 0) {
if (jt != NULL) {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ", monitor=" INTPTR_FORMAT
": free per-thread monitor must have NULL _header "
"field: _header=" INTPTR_FORMAT, p2i(jt), p2i(n),
n->header().value());
*error_cnt_p = *error_cnt_p + 1;
}
}
if (n->object() != NULL) {
if (jt != NULL) {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ", monitor=" INTPTR_FORMAT
": free per-thread monitor must have NULL _object "
"field: _object=" INTPTR_FORMAT, p2i(jt), p2i(n),
p2i(n->object()));
} else {
out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": free global monitor "
"must have NULL _object field: _object=" INTPTR_FORMAT,
p2i(n), p2i(n->object()));
}
*error_cnt_p = *error_cnt_p + 1;
}
}
// Lock the next ObjectMonitor for traversal and unlock the current
// ObjectMonitor. Returns the next ObjectMonitor if there is one.
// Otherwise returns NULL (after unlocking the current ObjectMonitor).
// This function is used by the various list walker functions to
// safely walk a list without allowing an ObjectMonitor to be moved
// to another list in the middle of a walk.
static ObjectMonitor* lock_next_for_traversal(ObjectMonitor* cur) {
assert(is_locked(cur), "cur=" INTPTR_FORMAT " must be locked", p2i(cur));
ObjectMonitor* next = unmarked_next(cur);
if (next == NULL) { // Reached the end of the list.
om_unlock(cur);
return NULL;
}
om_lock(next); // Lock next before unlocking current to keep
om_unlock(cur); // from being by-passed by another thread.
return next;
}
// Check the global free list and count; log the results of the checks.
void ObjectSynchronizer::chk_global_free_list_and_count(outputStream * out,
int *error_cnt_p) {
int chk_om_free_count = 0;
ObjectMonitor* cur = NULL;
if ((cur = get_list_head_locked(&om_list_globals._free_list)) != NULL) {
// Marked the global free list head so process the list.
while (true) {
chk_free_entry(NULL /* jt */, cur, out, error_cnt_p);
chk_om_free_count++;
cur = lock_next_for_traversal(cur);
if (cur == NULL) {
break;
}
}
}
int l_free_count = Atomic::load(&om_list_globals._free_count);
if (l_free_count == chk_om_free_count) {
out->print_cr("global_free_count=%d equals chk_om_free_count=%d",
l_free_count, chk_om_free_count);
} else {
// With fine grained locks on om_list_globals._free_list, it
// is possible for an ObjectMonitor to be prepended to
// om_list_globals._free_list after we started calculating
// chk_om_free_count so om_list_globals._free_count may not
// match anymore.
out->print_cr("WARNING: global_free_count=%d is not equal to "
"chk_om_free_count=%d", l_free_count, chk_om_free_count);
}
}
// Check the global wait list and count; log the results of the checks.
void ObjectSynchronizer::chk_global_wait_list_and_count(outputStream * out,
int *error_cnt_p) {
int chk_om_wait_count = 0;
ObjectMonitor* cur = NULL;
if ((cur = get_list_head_locked(&om_list_globals._wait_list)) != NULL) {
// Marked the global wait list head so process the list.
while (true) {
// Rules for om_list_globals._wait_list are the same as for
// om_list_globals._free_list:
chk_free_entry(NULL /* jt */, cur, out, error_cnt_p);
chk_om_wait_count++;
cur = lock_next_for_traversal(cur);
if (cur == NULL) {
break;
}
}
}
if (Atomic::load(&om_list_globals._wait_count) == chk_om_wait_count) {
out->print_cr("global_wait_count=%d equals chk_om_wait_count=%d",
Atomic::load(&om_list_globals._wait_count), chk_om_wait_count);
} else {
out->print_cr("ERROR: global_wait_count=%d is not equal to "
"chk_om_wait_count=%d",
Atomic::load(&om_list_globals._wait_count), chk_om_wait_count);
*error_cnt_p = *error_cnt_p + 1;
}
}
// Check the global in-use list and count; log the results of the checks.
void ObjectSynchronizer::chk_global_in_use_list_and_count(outputStream * out,
int *error_cnt_p) {
int chk_om_in_use_count = 0;
ObjectMonitor* cur = NULL;
if ((cur = get_list_head_locked(&om_list_globals._in_use_list)) != NULL) {
// Marked the global in-use list head so process the list.
while (true) {
chk_in_use_entry(NULL /* jt */, cur, out, error_cnt_p);
chk_om_in_use_count++;
cur = lock_next_for_traversal(cur);
if (cur == NULL) {
break;
}
}
}
int l_in_use_count = Atomic::load(&om_list_globals._in_use_count);
if (l_in_use_count == chk_om_in_use_count) {
out->print_cr("global_in_use_count=%d equals chk_om_in_use_count=%d",
l_in_use_count, chk_om_in_use_count);
} else {
// With fine grained locks on the monitor lists, it is possible for
// an exiting JavaThread to put its in-use ObjectMonitors on the
// global in-use list after chk_om_in_use_count is calculated above.
out->print_cr("WARNING: global_in_use_count=%d is not equal to chk_om_in_use_count=%d",
l_in_use_count, chk_om_in_use_count);
}
}
// Check an in-use monitor entry; log any errors.
void ObjectSynchronizer::chk_in_use_entry(JavaThread* jt, ObjectMonitor* n,
outputStream * out, int *error_cnt_p) {
if (n->header().value() == 0) {
if (jt != NULL) {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ", monitor=" INTPTR_FORMAT
": in-use per-thread monitor must have non-NULL _header "
"field.", p2i(jt), p2i(n));
} else {
out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": in-use global monitor "
"must have non-NULL _header field.", p2i(n));
}
*error_cnt_p = *error_cnt_p + 1;
}
if (n->object() == NULL) {
if (jt != NULL) {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ", monitor=" INTPTR_FORMAT
": in-use per-thread monitor must have non-NULL _object "
"field.", p2i(jt), p2i(n));
} else {
out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": in-use global monitor "
"must have non-NULL _object field.", p2i(n));
}
*error_cnt_p = *error_cnt_p + 1;
}
const oop obj = (oop)n->object();
const markWord mark = obj->mark();
if (!mark.has_monitor()) {
if (jt != NULL) {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ", monitor=" INTPTR_FORMAT
": in-use per-thread monitor's object does not think "
"it has a monitor: obj=" INTPTR_FORMAT ", mark="
INTPTR_FORMAT, p2i(jt), p2i(n), p2i(obj), mark.value());
} else {
out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": in-use global "
"monitor's object does not think it has a monitor: obj="
INTPTR_FORMAT ", mark=" INTPTR_FORMAT, p2i(n),
p2i(obj), mark.value());
}
*error_cnt_p = *error_cnt_p + 1;
}
ObjectMonitor* const obj_mon = mark.monitor();
if (n != obj_mon) {
if (jt != NULL) {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ", monitor=" INTPTR_FORMAT
": in-use per-thread monitor's object does not refer "
"to the same monitor: obj=" INTPTR_FORMAT ", mark="
INTPTR_FORMAT ", obj_mon=" INTPTR_FORMAT, p2i(jt),
p2i(n), p2i(obj), mark.value(), p2i(obj_mon));
} else {
out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": in-use global "
"monitor's object does not refer to the same monitor: obj="
INTPTR_FORMAT ", mark=" INTPTR_FORMAT ", obj_mon="
INTPTR_FORMAT, p2i(n), p2i(obj), mark.value(), p2i(obj_mon));
}
*error_cnt_p = *error_cnt_p + 1;
}
}
// Check the thread's free list and count; log the results of the checks.
void ObjectSynchronizer::chk_per_thread_free_list_and_count(JavaThread *jt,
outputStream * out,
int *error_cnt_p) {
int chk_om_free_count = 0;
ObjectMonitor* cur = NULL;
if ((cur = get_list_head_locked(&jt->om_free_list)) != NULL) {
// Marked the per-thread free list head so process the list.
while (true) {
chk_free_entry(jt, cur, out, error_cnt_p);
chk_om_free_count++;
cur = lock_next_for_traversal(cur);
if (cur == NULL) {
break;
}
}
}
int l_om_free_count = Atomic::load(&jt->om_free_count);
if (l_om_free_count == chk_om_free_count) {
out->print_cr("jt=" INTPTR_FORMAT ": om_free_count=%d equals "
"chk_om_free_count=%d", p2i(jt), l_om_free_count, chk_om_free_count);
} else {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ": om_free_count=%d is not "
"equal to chk_om_free_count=%d", p2i(jt), l_om_free_count,
chk_om_free_count);
*error_cnt_p = *error_cnt_p + 1;
}
}
// Check the thread's in-use list and count; log the results of the checks.
void ObjectSynchronizer::chk_per_thread_in_use_list_and_count(JavaThread *jt,
outputStream * out,
int *error_cnt_p) {
int chk_om_in_use_count = 0;
ObjectMonitor* cur = NULL;
if ((cur = get_list_head_locked(&jt->om_in_use_list)) != NULL) {
// Marked the per-thread in-use list head so process the list.
while (true) {
chk_in_use_entry(jt, cur, out, error_cnt_p);
chk_om_in_use_count++;
cur = lock_next_for_traversal(cur);
if (cur == NULL) {
break;
}
}
}
int l_om_in_use_count = Atomic::load(&jt->om_in_use_count);
if (l_om_in_use_count == chk_om_in_use_count) {
out->print_cr("jt=" INTPTR_FORMAT ": om_in_use_count=%d equals "
"chk_om_in_use_count=%d", p2i(jt), l_om_in_use_count,
chk_om_in_use_count);
} else {
out->print_cr("ERROR: jt=" INTPTR_FORMAT ": om_in_use_count=%d is not "
"equal to chk_om_in_use_count=%d", p2i(jt), l_om_in_use_count,
chk_om_in_use_count);
*error_cnt_p = *error_cnt_p + 1;
}
}
// Log details about ObjectMonitors on the in-use lists. The 'BHL'
// flags indicate why the entry is in-use, 'object' and 'object type'
// indicate the associated object and its type.
void ObjectSynchronizer::log_in_use_monitor_details(outputStream * out) {
stringStream ss;
if (Atomic::load(&om_list_globals._in_use_count) > 0) {
out->print_cr("In-use global monitor info:");
out->print_cr("(B -> is_busy, H -> has hash code, L -> lock status)");
out->print_cr("%18s %s %18s %18s",
"monitor", "BHL", "object", "object type");
out->print_cr("================== === ================== ==================");
ObjectMonitor* cur = NULL;
if ((cur = get_list_head_locked(&om_list_globals._in_use_list)) != NULL) {
// Marked the global in-use list head so process the list.
while (true) {
const oop obj = (oop) cur->object();
const markWord mark = cur->header();
ResourceMark rm;
out->print(INTPTR_FORMAT " %d%d%d " INTPTR_FORMAT " %s", p2i(cur),
cur->is_busy() != 0, mark.hash() != 0, cur->owner() != NULL,
p2i(obj), obj->klass()->external_name());
if (cur->is_busy() != 0) {
out->print(" (%s)", cur->is_busy_to_string(&ss));
ss.reset();
}
out->cr();
cur = lock_next_for_traversal(cur);
if (cur == NULL) {
break;
}
}
}
}
out->print_cr("In-use per-thread monitor info:");
out->print_cr("(B -> is_busy, H -> has hash code, L -> lock status)");
out->print_cr("%18s %18s %s %18s %18s",
"jt", "monitor", "BHL", "object", "object type");
out->print_cr("================== ================== === ================== ==================");
for (JavaThreadIteratorWithHandle jtiwh; JavaThread *jt = jtiwh.next(); ) {
ObjectMonitor* cur = NULL;
if ((cur = get_list_head_locked(&jt->om_in_use_list)) != NULL) {
// Marked the global in-use list head so process the list.
while (true) {
const oop obj = (oop) cur->object();
const markWord mark = cur->header();
ResourceMark rm;
out->print(INTPTR_FORMAT " " INTPTR_FORMAT " %d%d%d " INTPTR_FORMAT
" %s", p2i(jt), p2i(cur), cur->is_busy() != 0,
mark.hash() != 0, cur->owner() != NULL, p2i(obj),
obj->klass()->external_name());
if (cur->is_busy() != 0) {
out->print(" (%s)", cur->is_busy_to_string(&ss));
ss.reset();
}
out->cr();
cur = lock_next_for_traversal(cur);
if (cur == NULL) {
break;
}
}
}
}
out->flush();
}
// Log counts for the global and per-thread monitor lists and return
// the population count.
int ObjectSynchronizer::log_monitor_list_counts(outputStream * out) {
int pop_count = 0;
out->print_cr("%18s %10s %10s %10s %10s",
"Global Lists:", "InUse", "Free", "Wait", "Total");
out->print_cr("================== ========== ========== ========== ==========");
int l_in_use_count = Atomic::load(&om_list_globals._in_use_count);
int l_free_count = Atomic::load(&om_list_globals._free_count);
int l_wait_count = Atomic::load(&om_list_globals._wait_count);
out->print_cr("%18s %10d %10d %10d %10d", "", l_in_use_count,
l_free_count, l_wait_count,
Atomic::load(&om_list_globals._population));
pop_count += l_in_use_count + l_free_count + l_wait_count;
out->print_cr("%18s %10s %10s %10s",
"Per-Thread Lists:", "InUse", "Free", "Provision");
out->print_cr("================== ========== ========== ==========");
for (JavaThreadIteratorWithHandle jtiwh; JavaThread *jt = jtiwh.next(); ) {
int l_om_in_use_count = Atomic::load(&jt->om_in_use_count);
int l_om_free_count = Atomic::load(&jt->om_free_count);
out->print_cr(INTPTR_FORMAT " %10d %10d %10d", p2i(jt),
l_om_in_use_count, l_om_free_count, jt->om_free_provision);
pop_count += l_om_in_use_count + l_om_free_count;
}
return pop_count;
}
#ifndef PRODUCT
// Check if monitor belongs to the monitor cache
// The list is grow-only so it's *relatively* safe to traverse
// the list of extant blocks without taking a lock.
int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
PaddedObjectMonitor* block = Atomic::load(&g_block_list);
while (block != NULL) {
assert(block->object() == CHAINMARKER, "must be a block header");
if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
address mon = (address)monitor;
address blk = (address)block;
size_t diff = mon - blk;
assert((diff % sizeof(PaddedObjectMonitor)) == 0, "must be aligned");
return 1;
}
// unmarked_next() is not needed with g_block_list (no locking
// used with block linkage _next_om fields).
block = (PaddedObjectMonitor*)block->next_om();
}
return 0;
}
#endif
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