linux-kernel - Re: [PATCH 1/2] sched: Deny self-issued __set_cpus_allowed_ptr() when migrate

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Date:   Tue, 13 Oct 2020 16:21:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        qais.yousef@....com, swood@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vincent.donnefort@....com, tj@...nel.org
Subject: Re: [PATCH 1/2] sched: Deny self-issued __set_cpus_allowed_ptr()
 when migrate_disable()

On Tue, Oct 13, 2020 at 04:15:08PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-10-13 15:01:15 [+0100], Valentin Schneider wrote:
> >   migrate_disable();
> >   set_cpus_allowed_ptr(current, {something excluding task_cpu(current)});
> >   affine_move_task(); <-- never returns
> > 
> > Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> > ---
> >  kernel/sched/core.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 4ccd1099adaa..7f4e38819de1 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2189,6 +2189,11 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> >  	if (!(flags & SCA_MIGRATE_ENABLE) && cpumask_equal(&p->cpus_mask, new_mask))
> >  		goto out;
> >  
> > +	if (p == current &&
> > +	    is_migration_disabled(p) &&
> > +	    !cpumask_test_cpu(task_cpu(p), new_mask))
> > +		ret = -EBUSY;
> > +
> 
> This shouldn't happen, right? The function may sleep so it shouldn't be
> entered with disabled migration. A WARN_ON might spot the bad caller.

So yeah, I like detecting the case but agree with bigeasy that an
additional WARN would make sense, lemme go add that.