linux-kernel - Re: [PATCH] mm: oom: Fix race condition between oom_badness and do

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Date:   Thu, 8 Mar 2018 10:21:26 +0530
From:   "Kohli, Gaurav" <gkohli@...eaurora.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        kirill.shutemov@...ux.intel.com, aarcange@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] mm: oom: Fix race condition between oom_badness and
 do_exit of task

On 3/8/2018 2:26 AM, David Rientjes wrote:

> On Wed, 7 Mar 2018, Gaurav Kohli wrote:
>
>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>> index 6fd9773..5f4cc4b 100644
>> --- a/mm/oom_kill.c
>> +++ b/mm/oom_kill.c
>> @@ -114,9 +114,11 @@ struct task_struct *find_lock_task_mm(struct task_struct *p)
>>   
>>   	for_each_thread(p, t) {
>>   		task_lock(t);
>> +		get_task_struct(t);
>>   		if (likely(t->mm))
>>   			goto found;
>>   		task_unlock(t);
>> +		put_task_struct(t);
>>   	}
>>   	t = NULL;
>>   found:
> We hold rcu_read_lock() here, so perhaps only do get_task_struct() before
> doing rcu_read_unlock() and we have a non-NULL t?

Here rcu_read_lock will not help, as our task may change due to below algo:

for_each_thread(p, t) {
  		task_lock(t);
+		get_task_struct(t);
  		if (likely(t->mm))
  			goto found;
  		task_unlock(t);
+		put_task_struct(t)


So only we can increase usage counter here only at the current task.

I have seen you new patch, that seems valid to me and it will resolve our issue.
Thanks for support.

Regards

Gaurav

>
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