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8214302: Allow safely calling is_unloading() on zombie nmethods

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Reviewed-by: kvn, pliden
This commit is contained in:
Erik Österlund 2018-12-05 11:01:44 +01:00
parent cb1aebae6e
commit e03d20eb41
1 changed files with 33 additions and 3 deletions
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36
src/hotspot/share/code/nmethod.cpp
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@ -1575,14 +1575,44 @@ bool nmethod::is_unloading() {
if (state_is_unloading) {
return true;
}
if (state_unloading_cycle == CodeCache::unloading_cycle()) {
uint8_t current_cycle = CodeCache::unloading_cycle();
if (state_unloading_cycle == current_cycle) {
return false;
}
// The IsUnloadingBehaviour is responsible for checking if there are any dead
// oops in the CompiledMethod, by calling oops_do on it.
state_unloading_cycle = CodeCache::unloading_cycle();
state_is_unloading = IsUnloadingBehaviour::current()->is_unloading(this);
state_unloading_cycle = current_cycle;
if (is_zombie()) {
// Zombies without calculated unloading epoch are never unloading due to GC.
// There are no races where a previously observed is_unloading() nmethod
// suddenly becomes not is_unloading() due to here being observed as zombie.
// With STW unloading, all is_alive() && is_unloading() nmethods are unlinked
// and unloaded in the safepoint. That makes races where an nmethod is first
// observed as is_alive() && is_unloading() and subsequently observed as
// is_zombie() impossible.
// With concurrent unloading, all references to is_unloading() nmethods are
// first unlinked (e.g. IC caches and dependency contexts). Then a global
// handshake operation is performed with all JavaThreads before finally
// unloading the nmethods. The sweeper never converts is_alive() && is_unloading()
// nmethods to zombies; it waits for them to become is_unloaded(). So before
// the global handshake, it is impossible for is_unloading() nmethods to
// racingly become is_zombie(). And is_unloading() is calculated for all is_alive()
// nmethods before taking that global handshake, meaning that it will never
// be recalculated after the handshake.
// After that global handshake, is_unloading() nmethods are only observable
// to the iterators, and they will never trigger recomputation of the cached
// is_unloading_state, and hence may not suffer from such races.
state_is_unloading = false;
} else {
state_is_unloading = IsUnloadingBehaviour::current()->is_unloading(this);
}
state = IsUnloadingState::create(state_is_unloading, state_unloading_cycle);