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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 #172

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0a5aee4
fix: move release fence in inner_enqueue()
cppmage Aug 15, 2026
ba8c662
Fence and Comments
cppmage Aug 24, 2026
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14 changes: 7 additions & 7 deletions readerwriterqueue.h
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
// ©2013-2020 Cameron Desrochers.
// ©2013-2020 Cameron Desrochers.
// Distributed under the simplified BSD license (see the license file that
// should have come with this header).

Expand Down Expand Up @@ -619,14 +619,14 @@ class MOODYCAMEL_MAYBE_ALIGN_TO_CACHELINE ReaderWriterQueue
newBlock->tail = newBlock->localTail = 1;

newBlock->next = tailBlock_->next.load();
tailBlock_->next = newBlock;

// Might be possible for the dequeue thread to see the new tailBlock->next
// *without* seeing the new tailBlock value, but this is OK since it can't
// advance to the next block until tailBlock is set anyway (because the only
// 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).
// Publish all writes to *newBlock before it becomes reachable via
// tailBlock_->next.
fence(memory_order_release);
tailBlock_->next = newBlock;

// Ensure that readers observing the new tailBlock also observe the
// preceding publication of tailBlock_->next.
fence(memory_order_release);
tailBlock = newBlock;

@cameron314 cameron314 Aug 23, 2026

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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.

@cppmage cppmage Aug 24, 2026

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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.

Yeap, that is right. We need to guarantee that tailBlock's data was prepared before changing it.
I'll restore the second fence and clarify the comment accordingly.

}
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