Re: [PATCH] mm: oom: Fix race condition between oom_badness and do_exit of task

[Kohli, Gaurav]

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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