[ruby-core:122979] [Ruby Bug#21342] Segfault: invalid keeping_mutexes when using Mutex in Thread then Fiber after GC

Issue #21342 has been updated by luke-gru (Luke Gruber).

Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: REQUIRED

I created a PR here: https://github.com/ruby/ruby/pull/14262

----------------------------------------
Bug #21342: Segfault: invalid keeping_mutexes when using Mutex in Thread then Fiber after GC
https://bugs.ruby-lang.org/issues/21342#change-114282

* 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: REQUIRED
----------------------------------------
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)


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