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Re: (call-process ...) hangs in emacs

Hi Ken,

On Aug  5 13:55, Ken Brown wrote:
> On 8/5/2014 9:58 AM, Corinna Vinschen wrote:
> >On Aug  5 08:21, Ken Brown wrote:
> >>=== modified file 'src/gmalloc.c'
> >>--- src/gmalloc.c       2014-03-04 19:02:49 +0000
> >>+++ src/gmalloc.c       2014-08-05 01:35:38 +0000
> >>@@ -490,8 +490,8 @@
> >>  }
> >>
> >>  #ifdef USE_PTHREAD
> >>-pthread_mutex_t _malloc_mutex = PTHREAD_MUTEX_INITIALIZER;
> >>-pthread_mutex_t _aligned_blocks_mutex = PTHREAD_MUTEX_INITIALIZER;
> >>+pthread_mutex_t _malloc_mutex;
> >>+pthread_mutex_t _aligned_blocks_mutex;
> >>  int _malloc_thread_enabled_p;
> >>
> >>  static void
> >>@@ -526,8 +526,11 @@
> >>       initialized mutexes when they are used first.  To avoid such a
> >>       situation, we initialize mutexes here while their use is
> >>       disabled in malloc etc.  */
> >>-  pthread_mutex_init (&_malloc_mutex, NULL);
> >>-  pthread_mutex_init (&_aligned_blocks_mutex, NULL);
> >>+  pthread_mutexattr_t attr1, attr2;
> >>+  pthread_mutexattr_settype (&attr1, PTHREAD_MUTEX_NORMAL);
> >>+  pthread_mutexattr_settype (&attr2, PTHREAD_MUTEX_NORMAL);
> >>+  pthread_mutex_init (&_malloc_mutex, &attr1);
> >>+  pthread_mutex_init (&_aligned_blocks_mutex, &attr2);
> >>    pthread_atfork (malloc_atfork_handler_prepare,
> >>                   malloc_atfork_handler_parent,
> >>                   malloc_atfork_handler_child);
> >>
> >>
> >>The first hunk avoids the double initialization, but I don't understand why
> >>the second hunk does anything.  Since PTHREAD_MUTEX_NORMAL is now the
> >>default, shouldn't calling pthread_mutex_init with NULL second argument be
> >>equivalent to my calls to pthread_mutexattr_settype?  Does this indicate a
> >>Cygwin bug, or am I misunderstanding something?
> >
> >AFAICS you're missing something.  Your pthread_mutexattr_t attr1, attr2
> >are not initialized.  They contain some random values, thus they are not
> >good objects.  The calls to pthread_mutexattr_settype as well as the
> >calls to pthread_mutex_init will fail with EINVAL, but you won't see it
> >due to missing error handling, and you end up without mutexes at all.
> >If you call pthread_mutexattr_init before calling
> >pthread_mutexattr_settype the situation shoul;d be the same as before.
> Thanks for catching my mistake.  Your earlier suggestion about explicitly
> setting the pthread_mutexes to be ERRORCHECK mutexes seems to fix the
> problem (as long as I remember to call pthread_mutexattr_init).  The revised
> patch is attached.  I went back to using both the static and dynamic
> initializations as in the original code, since you said that's harmless.

I'm glad to read that, but I'm still a little bit concerned.  If your
code works with ERRORCHECK mutexes but hangs with NORMAL mutexes, you
*might* miss an error case.

I'd suggest to tweak the pthread_mutex_lock/unlock calls and log the
threads calling it.  It looks like the same thread calls malloc from
malloc for some reason and it might be interesting to learn how that
happens and if it's really ok in this scenario, because it seems to
be unexpected by the code.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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