Narrow deadlock check for sibling fibers locking mutex - #9226
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headius merged 2 commits intoFeb 10, 2026
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* Only raise deadlock error when two different fibers from the same thread attempt to lock (jruby#9218). * Store the real parent thread as the locking thread for deadlock checks (fixes second case added in spec). Fixes jruby#9218
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In jruby#9226 a user discovered that thread interrupt during an attempt to lock a mutex may render that mutex unusable from any other fiber, even if it did not successfully lock. This spec tries to trigger that state by having one thread loop while triggering a raise interrupt on another thread that repeatedly locks a mutex from within fibers. This may not always fail when it is broken due to the timing issues involved in triggering the interrupt after the lock was acquired but before interrupts have been handled by the fiber, but 10x seems sufficient to trigger the bug on JRuby.
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Fix and specs for when multiple sibling fibers from the same thread attempt to lock the same mutex.
Additional fix to cover the unreported case of two separate fibers both locking (original case from #9218 was parent thread locks and then a fiber locks). This is the second case added in the specs.