[6.2][Concurrency] Fix alreadyLocked in withStatusRecordLock. #81206
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If the preloaded status is locked, then we need to reload it in order to distinguish between the current thread holding the lock and another thread holding the lock. Without this, if another thread holds the lock, then we won't set the is-locked bit. We'll still actually hold the lock, but other threads may perform operations locklessly if the bit is not set, which can cause a crash. By reloading status in that case, we ensure that the bit is always set correctly. This manifested as crashes in task cancellation but could cause other task-related issues as well. Also remove an assert of !isStatusRecordLocked() in AsyncTask::complete(). We allow other threads to access tasks and take the lock for things like cancellation, so the lock may legitimately be held at that point. rdar://150327908 (cherry picked from commit 325b66a)
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@swift-ci please test |
al45tair
approved these changes
Apr 30, 2025
tbkka
approved these changes
Apr 30, 2025
DougGregor
approved these changes
Apr 30, 2025
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Explanation: A locking bug can cause concurrent mutation of task records, resulting in crashes during concurrent task cancellation.
Scope: Affects Swift Concurrency code that cancels tasks.
Issue: rdar://150327908
Risk: Low, the problematic case now reloads the is-locked flag from the task with the lock held instead of using the existing value, which is definitely more correct. No other functional changes.
Testing: Added a concurrent cancellation test that verifies this fix.
Reviewer: @al45tair
Cherry-pick #81205 to
release/6.2.If the preloaded status is locked, then we need to reload it in order to distinguish between the current thread holding the lock and another thread holding the lock. Without this, if another thread holds the lock, then we won't set the is-locked bit. We'll still actually hold the lock, but other threads may perform operations locklessly if the bit is not set, which can cause a crash. By reloading status in that case, we ensure that the bit is always set correctly.
This manifested as crashes in task cancellation but could cause other task-related issues as well.
Also remove an assert of !isStatusRecordLocked() in AsyncTask::complete(). We allow other threads to access tasks and take the lock for things like cancellation, so the lock may legitimately be held at that point.
rdar://150327908