Bug #21959: rb_internal_thread_event_hooks_rw_lock is not reinitialized after fork causing deadlocks

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 4.0.1
• Backport: 3.2: WONTFIX, 3.3: DONE, 3.4: REQUIRED, 4.0: REQUIRED

[anmarchenko_datadog (Andrey Marchenko)]

Ruby's GVL Instrumentation API uses a read-write lock (rb_internal_thread_event_hooks_rw_lock) to protect the list of thread event hooks:

• Read lock — acquired on every GVL transition to iterate and call hook callbacks (rb_thread_execute_hooks)
• Write lock — acquired when adding/removing hooks (rb_internal_thread_add_event_hook, rb_internal_thread_remove_event_hook)

After fork(), Ruby reinitializes several internal locks (e.g. vm->ractor.sched.lock, timer_th.waiting_lock), but not rb_internal_thread_event_hooks_rw_lock. This wasn't added with the GVL Instrumentation API.

The full reproducer is available here: https://github.com/anmarchenko/ruby-locks-fork-bug

Deadlock sequence

1. Parent process has thread event hooks registered (e.g. by a profiler like dd-trace-rb)
2. Multiple threads run concurrently, causing GVL transitions — each transition acquires a read lock on the rwlock
3. fork() happens while a thread holds the read lock
4. In the child, only the forking thread survives — the thread that held the lock is gone, but the lock state is copied as-is
5. Child tries to add or remove a hook → needs write lock → blocks forever on a lock that will never be released
6. Deadlock

Impact

This affects any Ruby C extension using the GVL Instrumentation API in combination with fork-based servers (Resque, Unicorn, Passenger, etc.). The original report comes from dd-trace-rb's profiler deadlocking Resque workers on Alpine Linux (musl libc): https://github.com/DataDog/dd-trace-rb/issues/4967

[anmarchenko_datadog (Andrey Marchenko)]

I am planning to open a PR with proposed fix soon

[anmarchenko_datadog (Andrey Marchenko)]

The PR is open: https://github.com/ruby/ruby/pull/16474

[byroot (Jean Boussier)]

• Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN to 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED, 4.0: REQUIRED

[anmarchenko_datadog (Andrey Marchenko)]

• Status changed from Open to Closed

Applied in changeset git|c8155822c460a5734d700cd468d306ca03b44ce4.

---

reinit rb_internal_thread_event_hooks_rw_lock at fork

[Bug #21959]

[hsbt (Hiroshi SHIBATA)]

• Backport changed from 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED, 4.0: REQUIRED to 3.2: WONTFIX, 3.3: DONE, 3.4: REQUIRED, 4.0: REQUIRED

ruby_3_3 75de62b79bb1af97a905b2dce0a8cf6ffd64b083 merged revision(s) c8155822c460a5734d700cd468d306ca03b44ce4.