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Date:	Mon, 20 Oct 2014 13:00:20 +0400
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Kirill Tkhai <tkhai@...dex.ru>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Vladimir Davydov" <vdavydov@...allels.com>
Subject: Re: [PATCH] sched/numa: fix unsafe get_task_struct() in
 task_numa_assign()

В Вс, 19/10/2014 в 21:24 +0200, Oleg Nesterov пишет:
> On 10/19, Kirill Tkhai wrote:
> >
> > 19.10.2014, 00:59, "Oleg Nesterov" <oleg@...hat.com>:
> >
> > >  No, I don't think this can work. Let's look at the current code:
> > >
> > >          rcu_read_lock();
> > >          cur = ACCESS_ONCE(dst_rq->curr);
> > >          if (cur->pid == 0) /* idle */
> > >
> > >  And any dereference, even reading ->pid is not safe. This memory can be
> > >  freed, unmapped, reused, etc.
> > >
> > >  Looks like, task_numa_compare() needs to take dst_rq->lock and get the
> > >  refernce first.
> >
> > Yeah, detection of idle is not save. If we reorder the checks almost all
> > problems will be gone. All except unmapping. JFI, is it possible with
> > such kernel structures as task_struct?
> 
> Yes, if DEBUG_PAGEALLOC. See kernel_map_pages() in arch/x86/mm/pageattr.c
> kernel_map_pages(enable => false) clears PAGE_PRESENT if slab returns the
> pages to system.

Thanks, Oleg!

> 
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1165,7 +1165,30 @@ static void task_numa_compare(struct task_numa_env *env,
> >
> >  	rcu_read_lock();
> >  	cur = ACCESS_ONCE(dst_rq->curr);
> > -	if (cur->pid == 0) /* idle */
> > +	/*
> > +	 * No need to move the exiting task, and this ensures that ->curr
> > +	 * wasn't reaped and thus get_task_struct() in task_numa_assign()
> > +	 * is safe; note that rcu_read_lock() can't protect from the final
> > +	 * put_task_struct() after the last schedule().
> > +	 */
> > +	if (cur->flags & PF_EXITING)
> > +		cur = NULL;
> 
> so this needs probe_kernel_read(&cur->flags).
> 
> > +	if (cur != ACCESS_ONCE(dst_rq->curr))
> > +		cur = NULL;
> 
> Yes, if this task_struct was freed in between we do not care if this memory
> was reused (except PF_EXITING can be false positive). If it was freed and
> now the same memory is ->curr again we know that delayed_put_task_struct()
> can't be called until we drop rcu lock, even if PF_EXITING is already set
> again.
> 
> I won't argue, but you need to convince Peter to accept this hack ;)

Just sent a new version with all of you suggestions :) Thanks!

> 
> > >  Or, perhaps, we need to change the rules to ensure that any "task_struct *"
> > >  pointer is rcu-safe. Perhaps we have more similar problems... I'd like to
> > >  avoid this if possible.
> >
> > RT tree has:
> >
> > https://git.kernel.org/cgit/linux/kernel/git/paulg/3.10-rt-patches.git/
> > tree/patches/sched-delay-put-task.patch
> 
> Yes, and this obviously implies more rcu callbacks in flight, and another
> gp before __put_task_struct(). but may be we will need to do this anyway...

Kirill

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