Ruby

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¶

# 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.