Fix TSAN warnings with AtomicFreeList #8270
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I do not think there was an actual error with AtomicFreeList, however TSAN detected a data race when concurrent pop() happened. In that case, there is indeed a race, where we can end-up reading data that is already corrupted by the concurrent pop. However, that situation is corrected by the following CAS. Somehow TSAN didn't see that. The fix is strange and consists in replacing: ``` auto pNext = storage[offset].next; ``` with ``` auto s = storage[offset]; auto pNext = s.next; ``` In this PR we also adjust the memory ordering to be less strong. i.e. we do not need `memory_order_seq_cst`, only the appropriate acquire or release semantic. In addition we also make `Node* next` a non-atomic variable again. It should have been, but was change to placate an older version of TSAN. BUGS=[377369108]
poweifeng
approved these changes
Nov 13, 2024
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I do not think there was an actual error with AtomicFreeList, however TSAN detected a data race when concurrent pop() happened. In that case, there is indeed a race, where we can end-up reading data that is already corrupted by the concurrent pop. However, that situation is corrected by the following CAS. Somehow TSAN didn't see that. The fix is strange and consists in replacing:
with
In this PR we also adjust the memory ordering to be less strong. i.e. we do not need
memory_order_seq_cst, only the appropriate acquire or release semantic.In addition we also make
Node* nexta non-atomic variable again. It should have been, but was change to placate an older version of TSAN.BUGS=[377369108]