linux-kernel - Re: [PATCH RFC] sched: Revert delayed_put_task

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Date:	Thu, 16 Oct 2014 11:43:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	Kirill Tkhai <tkhai@...dex.ru>, Oleg Nesterov <oleg@...hat.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>
Subject: Re: [PATCH RFC] sched: Revert delayed_put_task_struct() and fix use
 after free

On Thu, Oct 16, 2014 at 12:16:44PM +0400, Kirill Tkhai wrote:
> В Чт, 16/10/2014 в 09:59 +0200, Peter Zijlstra пишет:
> > On Thu, Oct 16, 2014 at 01:46:07AM +0400, Kirill Tkhai wrote:
> > > > --- x/kernel/sched/fair.c
> > > > +++ x/kernel/sched/fair.c
> > > > @@ -1165,7 +1165,7 @@ static void task_numa_compare(struct tas
> > > >  
> > > >  	rcu_read_lock();
> > > >  	cur = ACCESS_ONCE(dst_rq->curr);
> > > > -	if (cur->pid == 0) /* idle */
> > > > +	if (is_idle_task(cur) || (curr->flags & PF_EXITING))
> > > >  		cur = NULL;
> > > >  
> > > >  	/*
> > > > 
> > > 
> > > Looks like, we have to use the same fix for task_numa_group().
> > 
> > Don't think so, task_numa_group() is only called from task_numa_fault()
> > which is on 'current' and neither idle and PF_EXITING should be
> > faulting.
> 
> Isn't task_numa_group() fully preemptible?

Not seeing how that is relevant.

> It seems cpu_rq(cpu)->curr is not always equal to p.

It should be afaict:

 task_numa_fault()
  p = current;

  task_numa_group(p, ..);

And like said, idle tasks and PF_EXITING task should never get (numa)
faults for they should never be touching userspace.
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