Deadlock with CountDownLatch

[Alex Kahn]

Hi,

I'm on Ruby 2.1.1 with Concurrent Ruby 1.0.0. When I run the code `CountDownLatch.new.wait` I get the following error:

irb(main):001:0> require 'concurrent'

=> true

irb(main):002:0> Concurrent::CountDownLatch.new.wait

fatal: No live threads left. Deadlock?

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/synchronization/mri_lockable_object.rb:43:in `sleep'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/synchronization/mri_lockable_object.rb:43:in `wait'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/synchronization/mri_lockable_object.rb:43:in `ns_wait'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/synchronization/abstract_lockable_object.rb:43:in `ns_wait_until'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/atomic/mutex_count_down_latch.rb:21:in `block in wait'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/synchronization/mri_lockable_object.rb:38:in `block in synchronize'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/synchronization/mri_lockable_object.rb:38:in `synchronize'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/synchronization/mri_lockable_object.rb:38:in `synchronize'

        from /Users/akahn/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/concurrent-ruby-1.0.0/lib/concurrent/atomic/mutex_count_down_latch.rb:21:in `wait'

        from (irb):2

        from /Users/akahn/.rbenv/versions/2.1.1/bin/irb:11:in `<main>'

Am I doing something wrong or is this a bug? I also tried with concurrent-ruby-ext but got the same result.

Thanks,

Alex 

[Jerry D'Antonio]

Alex,

You are using the latch wrong.

A CountDownLatch is a synchronization object shared between threads. One thread waits on the latch while another thread counts down. The `#wait` method block until the latch count reaches zero. Since you only have one thread (the main thread) you are hopelessly blocking the main thread forever. When Ruby detects the deadlock it raises an exception. This is the expected behavior for both CountDownLatch and Ruby.

Many good examples of how CountDownLatch is used can be found within the concurrent-ruby test suite. It's also used a few places in Rails 5 and in the Sucker Punch gem (both the library and its tests). One good example is here: https://github.com/ruby-concurrency/concurrent-ruby/blob/master/spec/concurrent/atomic/count_down_latch_spec.rb#L59

Best regards,

Jerry

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[Alex Kahn]

Thanks, Jerry. I was indeed trying to block the main thread indefinitely. I didn't realize that Ruby would detect this as a deadlock and raise an error!

Cheers,

Alex

[Jerry D'Antonio]

Why are you blocking the main thread when there are no other threads running? MRI will only raise the deadlock error if there are no other threads. If there are other threads running then you can block the main thread indefinitely without error (until the last other thread dies, that is). But a blocked main thread without other threads is basically a dead program so I'm confused as to what you are trying to accomplish. Can you describe what you are trying to do in your program?

PS: I post that response in Gitter, too.

Best regards,

Jerry