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6948537: CMS: BOT walkers observe out-of-thin-air zeros on sun4v sparc/CMT

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On sun4v/CMT avoid use of memset() in BOT updates so as to prevent concurrent BOT readers from seeing the phantom zeros arising from memset()'s use of BIS.

Reviewed-by: jmasa, johnc, minqi, poonam, tonyp
This commit is contained in:
Y. Srinivas Ramakrishna 2010-05-03 10:24:51 -07:00
parent 99196ff9ca
commit 762f60d343
4 changed files with 44 additions and 2 deletions
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28
hotspot/src/share/vm/memory/blockOffsetTable.hpp
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@ -140,14 +140,38 @@ class BlockOffsetSharedArray: public CHeapObj {
"right address out of range");
assert(left < right, "Heap addresses out of order");
size_t num_cards = pointer_delta(right, left) >> LogN_words;
memset(&_offset_array[index_for(left)], offset, num_cards);
// Below, we may use an explicit loop instead of memset()
// because on certain platforms memset() can give concurrent
// readers "out-of-thin-air," phantom zeros; see 6948537.
if (UseMemSetInBOT) {
memset(&_offset_array[index_for(left)], offset, num_cards);
} else {
size_t i = index_for(left);
const size_t end = i + num_cards;
for (; i < end; i++) {
_offset_array[i] = offset;
}
}
}
void set_offset_array(size_t left, size_t right, u_char offset) {
assert(right < _vs.committed_size(), "right address out of range");
assert(left <= right, "indexes out of order");
size_t num_cards = right - left + 1;
memset(&_offset_array[left], offset, num_cards);
// Below, we may use an explicit loop instead of memset
// because on certain platforms memset() can give concurrent
// readers "out-of-thin-air," phantom zeros; see 6948537.
if (UseMemSetInBOT) {
memset(&_offset_array[left], offset, num_cards);
} else {
size_t i = left;
const size_t end = i + num_cards;
for (; i < end; i++) {
_offset_array[i] = offset;
}
}
}
void check_offset_array(size_t index, HeapWord* high, HeapWord* low) const {