From: MenTaLguY <mental@...>
Date: 2008-02-21T03:33:23+09:00
Subject: Re: Ruby 1.9, threads and FreeBSD 5

On Wed, 20 Feb 2008 22:09:59 +0900, Eric Jacoboni <jaco@neottia.net> wrote:
> 1.upto(10) do   # 10 threads ping
>   Thread.new do
>      mutex.synchronize do
>        pong.wait(mutex)
>        puts("Ping...")
>        ping.signal
>      end
>    end
> end
> 
> pong.signal   # Go!

Your code is buggy:  there is a race condition such that the initial #signal
can get called before any corresponding #waits.  That it superficially appeared
to work consistently with 1.8 was an accident of thread scheduling.  Adding
a 'sleep' beforehand, as another poster suggested, makes it more likely to
work but does not actually fix the bug.

The correct approach is to use a synchronization primitive which queues
notifications, for example a semaphore or a queue, rather than a condition
variable.  A queued notification can't get "missed" like this.

-mental