From: "jeremyevans0 (Jeremy Evans) via ruby-core" Date: 2023-06-12T02:15:34+00:00 Subject: [ruby-core:113876] [Ruby master Feature#19717] `ConditionVariable#signal` is not fair when the wakeup is consistently spurious. Issue #19717 has been updated by jeremyevans0 (Jeremy Evans). Tracker changed from Bug to Feature Backport deleted (3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN) Based on the thorough analysis by @kjtsanaktsidis, this doesn't appear to be a bug in Ruby, but rather common behavior of condition variable across implementations. The idea of `ConditionVariable#wait` taking a block to address the spurious wakeup issue seems interesting to me, but that would be a feature request. ---------------------------------------- Feature #19717: `ConditionVariable#signal` is not fair when the wakeup is consistently spurious. https://bugs.ruby-lang.org/issues/19717#change-103525 * 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/