From: Oleg Nesterov <oleg@redhat.com>
To: Kirill Tkhai <ktkhai@parallels.com>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>,
Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: introduce task_rcu_dereference?
Date: Thu, 23 Oct 2014 20:15:56 +0200 [thread overview]
Message-ID: <20141023181556.GA2740@redhat.com> (raw)
In-Reply-To: <20141022222330.GA28423@redhat.com>
On 10/23, Oleg Nesterov wrote:
>
> Damn.
Yes.
> On 10/22, Oleg Nesterov wrote:
> >
> > +struct task_struct *task_rcu_dereference(struct task_struct **ptask)
> > +{
> > + struct task_struct *task;
> > + struct sighand_struct *sighand;
> > +
> > + task = rcu_dereference(*ptask);
> > + if (!task)
> > + return NULL;
> > +
> > + /* If it fails the check below must fail too */
> > + probe_slab_address(&task->sighand, sighand);
> > + /*
> > + * Pairs with atomic_dec_and_test() in put_task_struct(task).
> > + * If we have read the freed/reused memory, we must see that
> > + * the pointer was updated. The caller might want to retry in
> > + * this case.
> > + */
> > + smp_rmb();
> > + if (unlikely(task != ACCESS_ONCE(*ptask)))
> > + return ERR_PTR(-EAGAIN);
>
> This is not exactly right. task == *ptask can be false positive.
>
> It can be freed, then resused (so that sighand != NULL can be false
> positive), then freed again, and then reused again as task_struct.
>
> This is not that bad, we still can safely use this task_struct, but
> the comment should be updated. Plus -EINVAL below can be wrong in
> this case although this minor.
Yes.
> Yeees, SLAB_DESTTROY_BY_RCU closes this race. Not sure why I'd like
> to avoid it, but I do ;)
Argh. I only meant that SLAB_DESTTROY_BY_RCU can make the comments
simpler. "closes this race" applies too "check below must fail too"
too. Sorry if I confused you.
"task == *ptask can be false positive" is true with or without
SLAB_DESTTROY_BY_RCU, and this needs a good comment. Yes, it can't
be reused twice, but still we can't 100% trust the "sighand != NULL"
check.
So let me repeat, SDBR can only turn probe_slab_address() into a plain
load.
But I can't think properly today, will try to recheck tomorrow and send
v2.
Oleg.
next prev parent reply other threads:[~2014-10-23 18:19 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-22 7:17 [PATCH v4] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Kirill Tkhai
2014-10-22 21:30 ` introduce task_rcu_dereference? Oleg Nesterov
2014-10-22 22:23 ` Oleg Nesterov
2014-10-23 18:15 ` Oleg Nesterov [this message]
2014-10-23 8:10 ` Kirill Tkhai
2014-10-23 18:18 ` Oleg Nesterov
2014-10-24 7:51 ` Kirill Tkhai
2014-10-27 19:53 ` [PATCH 0/3] introduce task_rcu_dereference() Oleg Nesterov
2014-10-27 19:54 ` [PATCH 1/3] probe_kernel_address() can use __probe_kernel_read() Oleg Nesterov
2014-10-27 19:54 ` [PATCH 2/3] introduce probe_slab_address() Oleg Nesterov
2014-10-27 19:21 ` Christoph Lameter
2014-10-28 5:44 ` Kirill Tkhai
2014-10-28 5:48 ` Kirill Tkhai
2014-10-28 15:01 ` Peter Zijlstra
2014-10-28 17:56 ` Kirill Tkhai
2014-10-28 18:00 ` Kirill Tkhai
2014-10-28 19:55 ` Oleg Nesterov
2014-10-28 20:12 ` Oleg Nesterov
2014-10-29 5:10 ` Kirill Tkhai
2014-10-27 19:54 ` [PATCH 3/3] introduce task_rcu_dereference() Oleg Nesterov
2014-10-28 6:22 ` Kirill Tkhai
2016-05-18 17:02 ` Peter Zijlstra
2016-05-18 18:23 ` Oleg Nesterov
2016-05-18 19:10 ` Peter Zijlstra
2016-05-18 19:57 ` Oleg Nesterov
2016-05-26 11:34 ` Peter Zijlstra
2016-06-03 10:49 ` [tip:sched/core] sched/fair: Use task_rcu_dereference() tip-bot for Oleg Nesterov
2016-06-03 10:48 ` [tip:sched/core] sched/api: Introduce task_rcu_dereference() and try_get_task_struct() tip-bot for Oleg Nesterov
2014-10-28 11:02 ` [tip:sched/core] sched/numa: Fix unsafe get_task_struct() in task_numa_assign() tip-bot for Kirill Tkhai
2014-11-08 3:48 ` [PATCH v4] sched/numa: fix " Sasha Levin
2014-11-09 14:07 ` Kirill Tkhai
2014-11-10 10:03 ` Peter Zijlstra
2014-11-10 15:48 ` Sasha Levin
2014-11-10 16:01 ` Peter Zijlstra
2014-11-16 9:50 ` [tip:sched/urgent] sched/numa: Avoid selecting oneself as swap target tip-bot for Peter Zijlstra
2014-11-10 16:03 ` [PATCH v4] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Peter Zijlstra
2014-11-10 16:09 ` Sasha Levin
2014-11-10 16:16 ` Peter Zijlstra
2014-11-10 16:10 ` Kirill Tkhai
2014-11-10 16:36 ` Kirill Tkhai
2014-11-10 16:44 ` Sasha Levin
2014-11-10 20:01 ` Kirill Tkhai
2014-11-12 9:49 ` Kirill Tkhai
2014-11-15 2:38 ` Sasha Levin
2014-11-18 17:30 ` Sasha Levin
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