From mboxrd@z Thu Jan  1 00:00:00 1970
From: gilles.chanteperdrix@xenomai.org (Gilles Chanteperdrix)
Date: Thu, 02 Aug 2012 22:02:05 +0200
Subject: doubts about switch_mm
In-Reply-To: <20120802194931.GY6802@n2100.arm.linux.org.uk>
References: <501AD023.3020204@xenomai.org>
 <20120802194931.GY6802@n2100.arm.linux.org.uk>
Message-ID: <501ADCBD.20907@xenomai.org>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On 08/02/2012 09:49 PM, Russell King - ARM Linux wrote:

> On Thu, Aug 02, 2012 at 09:08:19PM +0200, Gilles Chanteperdrix wrote:
>> switch_mm code, as of 3.5 contains:
>>
>> 1   if (!cpumask_test_and_set_cpu(cpu, mm_cpumask(next)) || prev != next) {  
>> 2           check_and_switch_context(next, tsk);                             
>> 3           if (cache_is_vivt())                                             
>> 4                   cpumask_clear_cpu(cpu, mm_cpumask(prev));                
>> 5   }                                                                        
>>
>> Line 1 seems to mean that maybe switch_mm is called with prev == next.
>> But then, what line 4 does is certainly wrong if prev == next.
> 
> Look at it more carefully.
> 
> If prev == next, then we're already running with *this* mm.  The bit
> in the CPU mask for this CPU will be set.
> 
> So, cpumask_test_and_set_cpu(cpu, mm_cpumask(next)) will be true, and
> because the condition is inverted, the first half of the if condition
> is false.
> 
> The second half is false, because prev == next.  So lines 2-4 will not
> be executed.
>


So, prev != next on line 1 is useless ?


-- 
                                                                Gilles.