From: "ioquatix (Samuel Williams) via ruby-core" Date: 2023-06-12T07:45:17+00:00 Subject: [ruby-core:113886] [Ruby master Feature#19717] `ConditionVariable#signal` is not fair when the wakeup is consistently spurious. Issue #19717 has been updated by ioquatix (Samuel Williams). > I think it's possible, with a bit of shuffling around in thread_sync.c, for the implementation to not actually remove the thread off the ConditionVariable's waitq until it's actually successfully acquired the resource; that way, if e.g. #signal is called twice, the two threads that are woken up will maintain their relative order in the waitq. Yes, I think that would be a good idea if we can do it. ---------------------------------------- Feature #19717: `ConditionVariable#signal` is not fair when the wakeup is consistently spurious. https://bugs.ruby-lang.org/issues/19717#change-103536 * Author: ioquatix (Samuel Williams) * Status: Open * Priority: Normal ---------------------------------------- For background, see this issue . It looks like `ConditionVariable#signal` is not fair, if the calling thread immediately reacquires the resource. I've given a detailed reproduction here as it's non-trivial: . Because the spurious wakeup occurs, the thread is pushed to the back of the waitq, which means any other waiting thread will acquire the resource, and that thread will perpetually be at the back of the queue. I believe the solution is to change `ConditionVarialbe#signal` should only remove the thread from the waitq if it's possible to acquire the lock. Otherwise it should be left in place, so that the order is retained, this should result in fair scheduling. -- https://bugs.ruby-lang.org/ ______________________________________________ ruby-core mailing list -- ruby-core@ml.ruby-lang.org To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org ruby-core info -- https://ml.ruby-lang.org/mailman3/postorius/lists/ruby-core.ml.ruby-lang.org/