Must read SequenceNumber before reading the Item (#78178)

Co-authored-by: vsadov <8218165+VSadov@users.noreply.github.com>
dotnet · Nov 14, 2022 · af314f3 · af314f3
1 parent 89161f4
commit af314f3
Showing 1 changed file with 4 additions and 6 deletions.
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Concurrent/ConcurrentQueue.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Concurrent/ConcurrentQueue.cs
@@ -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.