Mutex can be locked by multiple threads, causing Monitor to sometimes hang
I have found a case where Monitor sometimes does not release the lock on its mutex where it should, because the internal counter is increased by another thread than the one holding it. From the trace it seems that
Mutex#lock succeeds while another thread has not yet unlocked it.
This seems to be triggered when a monitor is accessed by a freshly spawned thread. I have attached a small test case (based on
monitor.rb) that should trigger the bug reliably and a trace that shows what goes wrong. The most important bits: (showing only
... #<Thread:0x007fea210a8fd0> c-return mutex_bug.rb:12 lock Mutex 0 #<Thread:0x007fea21098c70> c-return mutex_bug.rb:12 lock Mutex 0 #<Thread:0x007fea210a8fd0> c-return mutex_bug.rb:22 unlock Mutex 0 #<Thread:0x007fea21061450> c-return mutex_bug.rb:12 lock Mutex #<Thread:0x007fea21098c70> 1
That last line means that the thread acquired a lock on the mutex, while an owner and count of 1 are already set (ie. it should not be lockable.)
(I first came across this bug on a Rails app when Rack::Timeout seemed to cause requests to hang when logging messages.)
Updated by jvdp (Jaap van der Plas) over 5 years ago
So now that I added a patch for
lib/monitor.rb I guess this should be assigned to Shugo Maeda (shugo) per https://bugs.ruby-lang.org/projects/ruby/wiki/MaintainersStdlib, could someone do that for me? Or should I make a new issue?