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Fixing memory ordering issue in ConcurrentQueue #78142

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Nov 10, 2022
Merged

Fixing memory ordering issue in ConcurrentQueue #78142

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10 changes: 4 additions & 6 deletions src/libraries/System.Private.CoreLib/src/System/Collections/Concurrent/ConcurrentQueue.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -528,13 +528,11 @@ private static T GetItemWhenAvailable(ConcurrentQueueSegment<T> segment, int i)

// If the expected sequence number is not yet written, we're still waiting for
// an enqueuer to finish storing it. Spin until it's there.
if ((segment._slots[i].SequenceNumber & segment._slotsMask) != expectedSequenceNumberAndMask)
SpinWait spinner = default;
// Must read SequenceNumber before reading Item, thus Volatile.Read
while ((Volatile.Read(ref segment._slots[i].SequenceNumber) & segment._slotsMask) != expectedSequenceNumberAndMask)
{
SpinWait spinner = default;
while ((Volatile.Read(ref segment._slots[i].SequenceNumber) & segment._slotsMask) != expectedSequenceNumberAndMask)
{
spinner.SpinOnce();
}
spinner.SpinOnce();
}

// Return the value from the slot.
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