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Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking?

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On Wednesday, 31 August 2016 13:54:36 UTC+5:30, Casper H.S. Dik  wrote:
> Sibin Thomas <sibinpthomas@gmail.com> writes:
> 
> >Situation:
> 
> > 1. Thread 1 currently owns the mutex.
> > 2. While Thread 1 retains ownership of the mutex, Thread 2 makes a request
> > for the same lock.
> > 3. Thread 1 unlocks the lock.
> 
> >At this juncture can another thread (say, Thread 3) swoop in and make a
> >request for the lock and acquire it (as shown in the image below)? Or does
> >POSIX guarantee that a mutex shall be acquired by a thread that is already
> >waiting for the said mutex at the moment of unlocking a mutex?
> 
> Yes.  Thread 3 can win.
> 
> >http://i.stack.imgur.com/yy937.png
> 
> >The man page for `pthread_mutex_unlock()` states that -=20
> 
> >> If there are threads blocked on the mutex object referenced by mutex
> >> when pthread_mutex_unlock() is called, resulting in the mutex becoming
> >> available, the scheduling policy shall determine which thread shall
> >> acquire the mutex.
> 
> >This seems to say that Thread 3 can not swoop in and acquire the mutex,
> >though I am not fully assured.
> 
> Why do you think the scheduling policy favors thread 2 over thread 3?
> 
> Thread 2 can be woken up but it may need to fetch its stack from
> swap or at least its caches might be call; a running thread 3
> has a lot of advantage over thread 2 and may swoop in and win.
> 
> Casper

I was influenced by the discussion here - http://austingroupbugs.net/view.php?id=609
I thought similar to pthread_signal() pthread_mutex_unlock() too does an "atomic unblock" of threads already in its 'Wait queue'

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Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Sibin Thomas <sibinpthomas@gmail.com> - 2016-08-30 22:39 -0700
  Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2016-08-31 08:24 +0000
    Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Sibin Thomas <sibinpthomas@gmail.com> - 2016-08-31 04:08 -0700
      Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2016-08-31 11:41 +0000
        Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Sibin Thomas <sibinpthomas@gmail.com> - 2016-08-31 10:08 -0700
      Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Kaz Kylheku <221-501-9011@kylheku.com> - 2016-08-31 18:08 +0000
        Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Sibin Thomas <sibinpthomas@gmail.com> - 2016-09-01 03:50 -0700
  Re: Is mutex ownership handed off strictly only to threads which have requested for the lock prior to unlocking? Kaz Kylheku <221-501-9011@kylheku.com> - 2016-08-31 17:59 +0000

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