Avoid internal exception on timeout of AsyncSemaphore request that races with disposal #924
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
…ces with disposal The repro is a very particularly timed race between a timer thread and disposal on another thread. As such, no test can reliably repro it except with careful scheduling under a debugger. ## The scenario 1. A contested enter request is made of the `AsyncSemaphore` that includes a timeout. 1. The timeout expires and `CancellationHandler` is invoked. The `WaiterInfo.Trigger.TrySetCanceled` call succeeds. This thread is now suspended, before it enters the lock on `AsyncSemaphore.syncObject`. 1. Another thread calls `AsyncSemaphore.Dispose()`, which runs to completion. 1. The timeout thread now resumes, acquires the lock and tries to remove its `WaiterInfo.Node` from the `LinkedList`, but the `LinkedList` was already cleared in `Dispose`, so an exception is thrown. The fix is simply to clear the `WaiterInfo.Node` property when that node no longer belongs to it. This causes the racing `CancellationHandler` to skip the code to remove and recycle the node. Fixes microsoft#906
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
The repro is a very particularly timed race between a timer thread and disposal on another thread. As such, no test can reliably repro it except with careful scheduling under a debugger.
The scenario
AsyncSemaphorethat includes a timeout.CancellationHandleris invoked. TheWaiterInfo.Trigger.TrySetCanceledcall succeeds. This thread is now suspended, before it enters the lock onAsyncSemaphore.syncObject.AsyncSemaphore.Dispose(), which runs to completion.WaiterInfo.Nodefrom theLinkedList, but theLinkedListwas already cleared inDispose, so an exception is thrown.The fix is simply to clear the
WaiterInfo.Nodeproperty when that node no longer belongs to it. This causes the racingCancellationHandlerto skip the code to remove and recycle the node.Fixes #906