Fix release fence ordering in inner_enqueue() to prevent size_approx() race on AArch64 - #172
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Fix release fence ordering in inner_enqueue() to prevent size_approx() race on AArch64#172cppmage wants to merge 1 commit into
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Move fence(memory_order_release) before both writes that publish a newly allocated block (tailBlock_->next and tailBlock), not just the second one. size_approx() reaches blocks via the next-chain and never reads tailBlock, so it wasn't covered by the existing fence placement. Fixes cameron314#171
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Aug 23, 2026
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Thanks for looking into this. It does seem like a real bug.
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| @@ -627,7 +634,6 @@ class MOODYCAMEL_MAYBE_ALIGN_TO_CACHELINE ReaderWriterQueue | |||
| // case where it could try to read the next is if it's already at the tailBlock, | |||
| // and it won't advance past tailBlock in any circumstance). | |||
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I think this comment was to justify the previous placement of the fence, and can be removed, since it clearly missed a case in its reasoning :)
| // and it won't advance past tailBlock in any circumstance). | ||
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| fence(memory_order_release); | ||
| tailBlock = newBlock; |
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I think we still need a release fence before setting tailBlock itself? Otherwise anything reading tailBlock might not see the latest value of tailBlock->next, which would be the opposite bug.
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Note that at the time this code was written, TSan generated false positives with lock-free data structures, which is why it wasn't used. I should probably try running under TSan again at some point, it's come a long way since then. |
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Fixes #171
What changed
Move
fence(memory_order_release)ininner_enqueue()'sCanAllocbranch so it precedes both writes that publish a newly allocated block
(
tailBlock_->next = newBlockandtailBlock = newBlock), instead ofonly the second one. See #171 for the full root-cause
analysis and repro.
Also expanded the comment at that call site to note that
size_approx()(and any otherBlock::next-chain traversal) dependson this ordering too, not just
try_dequeue().Testing
size_approxstress test run in a loop (1000 iterations).same loop (1000 iterations).
size_approxstress test has other pre-existingassertion failures, unrelated to this change and not addressed here.