Issue #21342 has been updated by byroot (Jean Boussier). Looks like we're missing a null check: ``` * frame #0: 0x0000000100349cb4 miniruby`thread_mutex_remove(thread=0x0000000000000000, mutex=0x0000600001147f00) at thread_sync.c:225:12 frame #1: 0x000000010033c1f0 miniruby`rb_mutex_unlock_th(mutex=0x0000600001147f00, th=0x0000000000000000, fiber=0x0000000122f0a9c0) at thread_sync.c:467:5 frame #2: 0x0000000100349a9c miniruby`mutex_free(ptr=0x0000600001147f00) at thread_sync.c:132:27 frame #3: 0x0000000100122bdc miniruby`rb_data_free(objspace=0x000000012380a200, obj=4349174720) at gc.c:1192:17 frame #4: 0x0000000100122638 miniruby`rb_gc_obj_free(objspace=0x000000012380a200, obj=4349174720) at gc.c:1371:14 ``` I suspec tthe thread was freed before the mutex. ---------------------------------------- Bug #21342: Segfault: invalid keeping_mutexes when using Mutex in Thread then Fiber after GC https://bugs.ruby-lang.org/issues/21342#change-113276 * Author: maciej.mensfeld (Maciej Mensfeld) * Status: Open * ruby -v: 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-linux] * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- Ruby crashes with a `[BUG] invalid keeping_mutexes error` when attempting to GC locked mutex that was used in a Thread within a Fiber context after garbage collection. The error indicates an attempt to unlock a mutex that is not locked, suggesting a state management issue with mutexes across Thread and Fiber boundaries. ## Ruby Version `ruby 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-linux]` ## Reproduce Process ``` ruby # segv.rb 5.times do m = Mutex.new Thread.new do m.synchronize do end end.join Fiber.new do GC.start m.lock end.resume end ``` 1. Save the above code to a file (e.g., `segv.rb`) 2. Run with `ruby segv.rb` 3. The crash occurs intermittently - sometimes it crashes immediately, sometimes it hangs, once in a while it works ## Actual Result The program crashes with the following error: ``` segv.rb: [BUG] invalid keeping_mutexes: Attempt to unlock a mutex which is not locked ruby 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-linux] ``` whole segfault in the attached txt file. Full crash backtrace shows the error originates from: - `rb_threadptr_unlock_all_locking_mutexes` in thread.c:450 - `rb_thread_terminate_all` in thread.c:467 The crash suggests an issue in mutex state management during thread termination. ## Expected Result The script should complete successfully without crashing. The mutex should be properly managed across Thread and Fiber contexts, and garbage collection should not interfere with mutex state. ---Files-------------------------------- crash.txt (23.4 KB) -- https://bugs.ruby-lang.org/