From: Oleg Nesterov <oleg@redhat.com>
To: Kirill Tkhai <tkhai@yandex.ru>
Cc: Kirill Tkhai <ktkhai@parallels.com>,
Peter Zijlstra <peterz@infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>
Subject: Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign()
Date: Sun, 19 Oct 2014 21:37:44 +0200 [thread overview]
Message-ID: <20141019193744.GA3097@redhat.com> (raw)
In-Reply-To: <20141019192437.GA842@redhat.com>
On 10/19, Oleg Nesterov wrote:
>
> On 10/19, Kirill Tkhai wrote:
> >
> > --- 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 ;)
>
> > > 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...
Forgot to mention... Or we can make task_struct_cachep SLAB_DESTROY_BY_RCU,
in this case ->curr (or any other "task_struct *" ponter) can not go away
under rcu_read_lock(). task_numa_compare() still needs the PF_EXITING check,
but we do not need to recheck ->curr or probe_kernel_read().
Oleg.
next prev parent reply other threads:[~2014-10-19 19:41 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-15 12:31 [PATCH RFC] sched: Revert delayed_put_task_struct() and fix use after free Kirill Tkhai
2014-10-15 15:06 ` Oleg Nesterov
2014-10-15 19:40 ` Oleg Nesterov
2014-10-15 21:46 ` Kirill Tkhai
2014-10-15 22:02 ` Kirill Tkhai
2014-10-16 7:59 ` Peter Zijlstra
2014-10-16 8:16 ` Kirill Tkhai
2014-10-16 9:43 ` Peter Zijlstra
2014-10-16 9:50 ` Kirill Tkhai
2014-10-16 9:51 ` Kirill Tkhai
2014-10-16 10:04 ` Kirill Tkhai
2014-10-17 21:34 ` Oleg Nesterov
2014-10-16 7:56 ` Peter Zijlstra
2014-10-16 8:01 ` Peter Zijlstra
2014-10-16 22:05 ` Oleg Nesterov
2014-10-17 21:36 ` [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Oleg Nesterov
2014-10-18 8:15 ` Kirill Tkhai
2014-10-18 8:33 ` Kirill Tkhai
2014-10-18 19:36 ` Peter Zijlstra
2014-10-18 21:18 ` Oleg Nesterov
2014-10-19 8:20 ` Kirill Tkhai
2014-10-18 20:56 ` Oleg Nesterov
2014-10-18 23:13 ` Kirill Tkhai
2014-10-19 19:24 ` Oleg Nesterov
2014-10-19 19:37 ` Oleg Nesterov [this message]
2014-10-19 19:43 ` Oleg Nesterov
2014-10-20 9:03 ` Kirill Tkhai
2014-10-20 9:13 ` Peter Zijlstra
2014-10-20 10:36 ` Kirill Tkhai
2014-10-20 9:00 ` Kirill Tkhai
2014-10-19 21:38 ` Peter Zijlstra
2014-10-20 8:56 ` Kirill Tkhai
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