Scoped cache broken due to operator== ABA problem #409
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bitfaster
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October 13, 2023 18:47
bitfaster
changed the title
added 2 commits
October 13, 2023 19:33
bitfaster
commented
Oct 14, 2023
| @@ -45,7 +45,7 @@ public bool TryCreateLifetime(out Lifetime<T> lifetime) | |||
| var oldRefCount = this.refCount; | |||
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| // If old ref count is 0, the scoped object has been disposed and there was a race. | |||
| if (this.isDisposed || oldRefCount.Count == 0) | |||
| if (IsDisposed || oldRefCount.Count == 0) | |||
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I think there is another potential race as follows:
- Thread A: check IsDisposed returns false
- Thread B: Dispose, IsDisposed now true
- Thread A: IncrementCount, return lifetime containing disposed value
I am not able to provoke this case with the soak test.
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See the wikipedia description for the ABA problem.
At some point, ReferenceCount was updated to implement IEquatable (possibly because code analysis said this is a good idea). This changed
operator ==from the default which tests whether two references of that type refer to the same object, to comparing for field equality.This broke the
Scopedclass. Specifically, this line and this line are now susceptible to the ABA problem because the equality operator is checking field values:if (oldRefCount == Interlocked.CompareExchange(ref this.refCount, oldRefCount.IncrementCopy(), oldRefCount))This issue does not affect SingletonCache, because it does not depend on
operator==oroperator!=.There was a further bug in
Scoped.DecrementReferenceCount()caused by reading a stale value ofthis.refCount.Countafter it was replaced by a compare exchange call.