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Date: Mon, 16 Mar 2015 05:40:15 +0200
From: Matthias Bonne <lemonlime51@gmail.com>
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To: Rabin Vincent <rabin@rab.in>
CC: Davidlohr Bueso <dave@stgolabs.net>, Yann Droneaud <ydroneaud@opteya.com>,
        kernelnewbies@kernelnewbies.org, linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Subject: Re: Question on mutex code
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>
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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.