From mboxrd@z Thu Jan  1 00:00:00 1970
From: lemonlime51@gmail.com (Matthias Bonne)
Date: Mon, 16 Mar 2015 05:40:15 +0200
Subject: Question on mutex code
In-Reply-To: <20150315221018.GA25881@debian>
References: <54F64E10.7050801@gmail.com> <1425992639.3991.11.camel@opteya.com>
	<5504BECB.50605@gmail.com> <1426381401.28068.68.camel@stgolabs.net>
	<1426381746.28068.70.camel@stgolabs.net>
	<5505FE53.1060807@gmail.com> <20150315221018.GA25881@debian>
Message-ID: <5506509F.1040108@gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

On 03/16/15 00:10, Rabin Vincent wrote:
> On Sun, Mar 15, 2015 at 11:49:07PM +0200, Matthias Bonne wrote:
>> So the counter is set to 1 before taking the spinlock, which I think
>> might cause the race. Did I miss something?
>
> Yes, you miss the fact that __mutex_slowpath_needs_to_unlock() is 0 for
> the CONFIG_DEBUG_MUTEXES case:
>
>   #ifdef CONFIG_DEBUG_MUTEXES
>   # include "mutex-debug.h"
>   # include <asm-generic/mutex-null.h>
>   /*
>    * Must be 0 for the debug case so we do not do the unlock outside of the
>    * wait_lock region. debug_mutex_unlock() will do the actual unlock in this
>    * case.
>    */
>   # undef __mutex_slowpath_needs_to_unlock
>   # define  __mutex_slowpath_needs_to_unlock()	0
>

Right, I overlooked this part. Thanks to both of you for the
clarifications.