Issue #20670 has been reported by jhawthorn (John Hawthorn). ---------------------------------------- Bug #20670: fork deadlocks in child process due to timer thread https://bugs.ruby-lang.org/issues/20670 * Author: jhawthorn (John Hawthorn) * Status: Open * ruby -v: ruby 3.3.4 (2024-07-09 revision be1089c8ec) [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- We've been seeing an occasional failure in the Rails CI related to a test which forks and I managed to reduce it to the following reproduction. ``` ruby Thread.new do loop { sleep 0.0001 } end 1000.times do pid = fork{} Process.waitpid(pid) rescue Exception Process.kill(:KILL, pid) raise end ``` This hangs on Ruby 3.3 and HEAD (_fairly_ reliably), but completes always on Ruby 3.2 In a debugger it seems like the timer thread acquires `vm->ractor.sched.lock` in the parent process just as the process is forking. The child process then ends up stuck inside of `thread_sched_atfork` when trying to acquire the same lock. I've opened https://github.com/ruby/ruby/pull/11356 with a fix -- https://bugs.ruby-lang.org/