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8269870: PS: Membar in PSPromotionManager::copy_unmarked_to_survivor_space could be relaxed #19764

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8269870: PS: Membar in PSPromotionManager::copy_unmarked_to_survivor_space could be relaxed #19764

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fb4a255
pgc-relax-forward
albertnetymk Jan 9, 2024
58e3807
review
albertnetymk Jun 26, 2024
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4 changes: 4 additions & 0 deletions src/hotspot/share/gc/parallel/psPromotionManager.inline.hpp
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Original file line number Diff line number Diff line change
@@ -254,6 +254,10 @@ inline oop PSPromotionManager::copy_unmarked_to_survivor_space(oop o,
ContinuationGCSupport::transform_stack_chunk(new_obj);

// Now we have to CAS in the header.
// Because the forwarding is done with memory_order_relaxed there is no
// ordering with the above copy. Clients that get the forwardee must not
// examine its contents without other synchronization, since the contents
// may not be up to date for them.
oop forwardee = o->forward_to_atomic(new_obj, test_mark, memory_order_relaxed);
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I think this needs a comment similar to what's in G1ParScanThreadState::do_copy_to_suvivor_space, e.g.

jdk/src/hotspot/share/gc/g1/g1ParScanThreadState.cpp

Lines 504 to 507 in f101e15

// Because the forwarding is done with memory_order_relaxed there is no
// ordering with the above copy. Clients that get the forwardee must not
// examine its contents without other synchronization, since the contents
// may not be up to date for them.

if (forwardee == nullptr) { // forwardee is null when forwarding is successful
// We won any races, we "own" this object.