From mboxrd@z Thu Jan  1 00:00:00 1970
From: gilles.chanteperdrix@xenomai.org (Gilles Chanteperdrix)
Date: Thu, 02 Aug 2012 22:05:16 +0200
Subject: doubts about switch_mm
In-Reply-To: <501ADCBD.20907@xenomai.org>
References: <501AD023.3020204@xenomai.org>
 <20120802194931.GY6802@n2100.arm.linux.org.uk> <501ADCBD.20907@xenomai.org>
Message-ID: <501ADD7C.40607@xenomai.org>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On 08/02/2012 10:02 PM, Gilles Chanteperdrix wrote:

> 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 ?
> 
> 

Ok, got it, sorry for the noise.

-- 
                                                                Gilles.