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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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10 changes: 8 additions & 2 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 @@ -619,6 +619,13 @@ class MOODYCAMEL_MAYBE_ALIGN_TO_CACHELINE ReaderWriterQueue
newBlock->tail = newBlock->localTail = 1;

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

// Publish all writes to *newBlock before it becomes reachable via either
// tailBlock_->next (walked by size_approx() and other chain traversals)
// or tailBlock itself (used by try_dequeue). Without this fence, on
// weakly-ordered architectures (e.g. AArch64) a reader could observe the
// updated `next` pointer before seeing newBlock's initialized fields.
fence(memory_order_release);
tailBlock_->next = newBlock;

// Might be possible for the dequeue thread to see the new tailBlock->next
Expand All @@ -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).

fence(memory_order_release);
tailBlock = newBlock;
}
else if (canAlloc == CannotAlloc) {
Expand Down