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Date:	Thu, 07 Aug 2014 09:30:53 -0600
From:	Eric Blake <eblake AT redhat DOT com>
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Subject:	Re: (call-process ...) hangs in emacs
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On 08/07/2014 05:51 AM, Ken Brown wrote:
>=20
> 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=3D858

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.

--=20
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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