From: Peter Zijlstra <peterz@infradead.org>
To: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Quentin Perret <qperret@google.com>,
linux-kernel@vger.kernel.org, aaron.lwe@gmail.com,
valentin.schneider@arm.com, mingo@kernel.org, pauld@redhat.com,
jdesfossez@digitalocean.com, naravamudan@digitalocean.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
juri.lelli@redhat.com, rostedt@goodmis.org, bsegall@google.com,
mgorman@suse.de, kernel-team@android.com, john.stultz@linaro.org
Subject: Re: Re: NULL pointer dereference in pick_next_task_fair
Date: Wed, 6 Nov 2019 18:27:37 +0100 [thread overview]
Message-ID: <20191106172737.GM5671@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20191106165437.GX4114@hirez.programming.kicks-ass.net>
On Wed, Nov 06, 2019 at 05:54:37PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 06, 2019 at 06:51:40PM +0300, Kirill Tkhai wrote:
> > > +#ifdef CONFIG_SMP
> > > + if (!rq->nr_running) {
> > > + /*
> > > + * Make sure task_on_rq_curr() fails, such that we don't do
> > > + * put_prev_task() + set_next_task() on this task again.
> > > + */
> > > + prev->on_cpu = 2;
> > > newidle_balance(rq, rf);
> >
> > Shouldn't we restore prev->on_cpu = 1 after newidle_balance()? Can't prev
> > become pickable again after newidle_balance() releases rq->lock, and we
> > take it again, so this on_cpu == 2 never will be cleared?
>
> Indeed so.
Oh wait, the way it was written this is not possible. Because
rq->nr_running == 0 and prev->on_cpu > 0 it means the current task is
going to sleep and cannot be woken back up.
But if I move the ->on_cpu=2 thing earlier, as I wrote I'd do, then yes,
we have to set it back to 1. Because in that case we can get here for a
spurious schedule and we'll pick the same task again.
next prev parent reply other threads:[~2019-11-06 17:27 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-28 17:46 NULL pointer dereference in pick_next_task_fair Quentin Perret
2019-10-28 21:49 ` Peter Zijlstra
2019-10-29 11:34 ` Peter Zijlstra
2019-10-29 11:50 ` Quentin Perret
2019-10-30 22:50 ` Ram Muthiah
2019-10-31 1:33 ` Valentin Schneider
2019-10-31 10:54 ` Valentin Schneider
2019-10-31 14:24 ` Valentin Schneider
2019-10-31 22:15 ` Valentin Schneider
2019-11-06 12:05 ` Peter Zijlstra
2019-11-06 13:08 ` Peter Zijlstra
2019-11-06 15:04 ` Qais Yousef
2019-11-06 16:57 ` Peter Zijlstra
2019-11-06 17:26 ` Qais Yousef
2019-11-06 15:51 ` Kirill Tkhai
2019-11-06 16:54 ` Peter Zijlstra
2019-11-06 17:27 ` Peter Zijlstra [this message]
2019-11-07 8:36 ` Kirill Tkhai
2019-11-07 13:26 ` Peter Zijlstra
2019-11-07 15:12 ` Kirill Tkhai
2019-11-07 15:42 ` Peter Zijlstra
2019-11-07 15:53 ` Kirill Tkhai
2019-11-07 15:38 ` Quentin Perret
2019-11-07 18:43 ` Peter Zijlstra
2019-11-07 19:27 ` Quentin Perret
2019-11-07 19:31 ` Peter Zijlstra
2019-11-07 19:42 ` Quentin Perret
2019-11-07 19:29 ` Peter Zijlstra
2019-11-08 11:02 ` Quentin Perret
2019-11-08 11:47 ` Valentin Schneider
2019-11-08 11:58 ` Quentin Perret
2019-11-08 12:00 ` Peter Zijlstra
2019-11-08 12:15 ` Quentin Perret
2019-11-08 12:35 ` Peter Zijlstra
2019-11-08 12:24 ` Peter Zijlstra
2019-11-08 11:55 ` Peter Zijlstra
2019-11-08 12:52 ` Peter Zijlstra
2019-11-07 16:09 ` Qais Yousef
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