From: Michael Malone <michael.malone@...>
Date: 2009-03-19T03:59:47+09:00
Subject: Re: does IO.read block?

Robert Klemme wrote:
> Here's another pastie that demonstrates a common error causing a deadlock:
>
> http://pastie.org/419533
>
> Cheers
>
> robert
>
>   

Thanks, but I have a new theory.  I broke everything down to be as 
atomic as possible, which included pulling in the Synchronised Queue 
from thread.rb and modifying it slightly to lock the mutex then unlock 
when necessary, so blocks were being passed as parameters as little as 
possible.  (As they are quite removed from being an atomic operation).  
And very nearly immediately, ruby's deadlock protection kicked in.  What 
happens is this: 

In the main thread, I create the jobs and put them on the queue.  
Depending on how things run (why it only happens sometimes) if the 
worker threads are scheduled more favourably, then they will complete, 
but not die (as they are blocking on queue, waiting for new jobs) 
meaning that each thread, except for the main thread is asleep.  I then 
call join() in the main thread meaning the main thread sleeps before the 
others have a chance to wakeup and finish, at which point ruby detects a 
deadlock, even though its next operation should be to wakeup a new 
thread.  At this point a "Bug Report: Unlocking mutex must be NULL" is 
generated by the interpreter and then hastily exits.

As soon as I have a smaller, specific case I will be filing a bug report. 
Thanks for all your help, but I think I have it now.

Michael

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