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Date:      Thu, 18 Jul 2013 11:13:31 -0400
From:      Joe Marcus Clarke <marcus@marcuscom.com>
To:        Daniel Eischen <deischen@freebsd.org>
Cc:        Koop Mast <kwm@rainbow-runner.nl>, freebsd-threads@freebsd.org
Subject:   Re: Mutexes and error checking
Message-ID:  <51E8061B.60906@marcuscom.com>
In-Reply-To: <Pine.GSO.4.64.1307181059460.22570@sea.ntplx.net>
References:  <51E71D4F.5030502@marcuscom.com> <Pine.GSO.4.64.1307181059460.22570@sea.ntplx.net>

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On 7/18/13 11:09 AM, Daniel Eischen wrote:
> On Wed, 17 Jul 2013, Joe Marcus Clarke wrote:
> 
>> It seems we might have a discrepancy between the way our pthread
>> implementation works compared to Linux.  If a mutex is set to NORMAL
>> type and one goes to unlock it, EPERM is returned unless the current
>> thread is the mutex owner.  While this sounds perfectly sane, it appears
>> Linux only returns EPERM if the mutex type is ERRORCHECK.
>>
>> We are seeing some problems in ported code because of this.  As a
>> suggestion, if people agree, would it be possible to emulate the
>> behavior of Linux and only return EPERM if the mutex is of type
>> ERRORCHECK or RECURSVIE?
> 
> First, any software that does that is broken.
> 
> Second, the POSIX spec seems to imply that an error is returned
> when a different thread tries to unlock an already locked mutex:
> 
>  
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.htm
> 
> 
> Is the mutex robust or not robust?  If not robust
> (PTHREAD_MUTEX_STALLED), then a PTHREAD_MUTEX_NORMAL mutex
> cannot be unlocked by any other thread than the owner.
> So, it would seem to be wrong to _not_ return an
> error when the mutex is not unlocked after
> pthread_mutex_unlock() returns.
> 

Don't get me wrong, I agree with you.  This behavior should result in
EPERM.  However, my comment was more on the portability side to maintain
parity with Linux in order to support the 3rd party code people wanting
to run on FreeBSD.  We can workaround it in some cases, but I was
floating up to you guys to perhaps create a broader workaround.

Joe



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