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Date: Thu, 07 Aug 2014 14:53:58 -0400
From: Ken Brown <kbrown AT cornell DOT edu>
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To: cygwin AT cygwin DOT com
Subject: Re: (call-process ...) hangs in emacs
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On 8/7/2014 11:30 AM, Eric Blake wrote:
> On 08/07/2014 05:51 AM, Ken Brown wrote:
>>
>> I think I found the problem with NORMAL mutexes.  emacs calls
>> pthread_atfork after initializing the mutexes, and the resulting
>> 'prepare' handler locks the mutexes.  (The parent and child handlers
>> unlock them.)  So when emacs calls fork, the mutexes are locked, and
>> shortly thereafter the Cygwin DLL calls calloc, leading to a deadlock.
>> Here's a gdb backtrace showing the sequence of calls:
>
> Arguably, that's an upstream bug in emacs.  POSIX has declared
> pthread_atfork to be fundamentally useless; it is broken by design,
> because you cannot use it for anything that is not async-signal-safe
> without risking deadlock.  And (except for sem_post()), NONE of the
> standardized locking functions are async-signal-safe.
>
> http://austingroupbugs.net/view.php?id=858
>
> That said, it would still be nice to support this, since even though the
> theory says it is broken, there are still lots of (broken)
> programs/libraries still trying to use it.

So what do you think emacs should do instead of using pthread_atfork? 
Or is it better to just remove it?  I don't know how likely it is that 
this would cause a problem.

Ken

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