Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751646AbaJRVAU (ORCPT ); Sat, 18 Oct 2014 17:00:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:31535 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751580AbaJRU7y (ORCPT ); Sat, 18 Oct 2014 16:59:54 -0400 Date: Sat, 18 Oct 2014 22:56:14 +0200 From: Oleg Nesterov To: Kirill Tkhai Cc: Kirill Tkhai , Peter Zijlstra , "linux-kernel@vger.kernel.org" , Ingo Molnar , Vladimir Davydov Subject: Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Message-ID: <20141018205614.GA15934@redhat.com> References: <1413376300.24793.55.camel@tkhai> <20141017213641.GB32576@redhat.com> <4323181413620101@web21o.yandex.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4323181413620101@web21o.yandex.ru> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/18, Kirill Tkhai wrote: > > 18.10.2014, 01:40, "Oleg Nesterov" : > > ... > > The > > task_struct itself can't go away, > > ... > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -1158,7 +1158,13 @@ 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 (is_idle_task(cur) || (cur->flags & PF_EXITING)) > > ?????????????????cur = NULL; > > > > ?????????/* > > Oleg, I've looked once again, and now it's not good for me. Ah. Thanks a lot Kirill for correcting me! I was looking at this rcu_read_lock() and I didn't even try to think what it can actually protect. Nothing. > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1165,7 +1165,21 @@ 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 (is_idle_task(cur) || (cur->flags & PF_EXITING)) > + cur = NULL; > + /* > + * Check once again to be sure curr is still on dst_rq. Even if > + * it points on a new task, which is using the memory of freed > + * cur, it's OK, because we've locked RCU before > + * delayed_put_task_struct() callback is called to put its struct. > + */ > + if (cur != ACCESS_ONCE(dst_rq->curr)) 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. 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. Hmm. I'll try to think more. Thanks! Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/