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Date:   Fri, 03 Nov 2017 01:44:44 +0800
From:   "Yang Shi" <yang.s@...baba-inc.com>
To:     Michal Hocko <mhocko@...nel.org>, mingo@...hat.com
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use in_atomic() in print_vma_addr()



On 11/2/17 12:57 AM, Michal Hocko wrote:
> On Thu 02-11-17 05:38:33, Yang Shi wrote:
>> commit 3e51f3c4004c9b01f66da03214a3e206f5ed627b
>> ("sched/preempt: Remove PREEMPT_ACTIVE unmasking off in_atomic()") makes
>> in_atomic() just check the preempt count, so it is not necessary to use
>> preempt_count() in print_vma_addr() any more. Replace preempt_count() to
>> in_atomic() which is a generic API for checking atomic context.
> 
> But why? Is there some general work to get rid of the direct preempt_count
> usage outside of the generic API?

I may not articulate it in the commit log, I would say "in_atomic" is 
*preferred* API for checking atomic context instead of preempt_count() 
which should be used for retrieving the preemption count value.

I would say there is not such general elimination work undergoing right 
now, but if we go through the kernel code, almost everywhere "in_atomic" 
is used for such use case already, except two places:

- print_vma_addr()
- debug_smp_processor_id()

Both came from Ingo long time ago before commit 
3e51f3c4004c9b01f66da03214a3e206f5ed627b ("sched/preempt: Remove 
PREEMPT_ACTIVE unmasking off in_atomic()"). But, after this commit was 
merged, I don't see why *not* use in_atomic() to follow the convention.

Thanks,
Yang

> 
>> Signed-off-by: Yang Shi <yang.s@...baba-inc.com>
>> ---
>>   mm/memory.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index a728bed..19b684e 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4460,7 +4460,7 @@ void print_vma_addr(char *prefix, unsigned long ip)
>>   	 * Do not print if we are in atomic
>>   	 * contexts (in exception stacks, etc.):
>>   	 */
>> -	if (preempt_count())
>> +	if (in_atomic())
>>   		return;
>>   
>>   	down_read(&mm->mmap_sem);
>> -- 
>> 1.8.3.1
> 

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