Re: [PATCH v2] sched: fix tsk->pi_lock isn't held when do_set_cpus_allowed()

[Wanpeng Li]

Hi Boqun,
On 8/28/15 2:33 PM, Boqun Feng wrote:
> Hi Wanpeng,
>
> On Fri, Aug 28, 2015 at 12:02:47PM +0800, Wanpeng Li wrote:
> <snip>
> > This patch fix it by following the rules for changing task_struct::cpus_allowed
> > w/ both pi_lock and rq->lock are held.
> >
> > Reported-by: kernel test robot <ying.huang@xxxxxxxxx>
> > Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > ---
> >   v1 -> v2:
> >    * fix the silly double lock stuff
> >    * follow the rules for changing task_struct::cpus_allowed
> >
> >   kernel/sched/core.c |   22 ++++++++++++++++++++++
> >   1 files changed, 22 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index b3386c6..8cf87e3 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -5186,6 +5186,27 @@ static void migrate_tasks(struct rq *dead_rq)
> >   		BUG_ON(!next);
> >   		next->sched_class->put_prev_task(rq, next);
> >   
> > +		/*
> > +		 * Rules for changing task_struct::cpus_allowed are holding
> > +		 * both pi_lock and rq->lock, such that holding either
> > +		 * stabilizes the mask.
> > +		 *
> > +		 * Drop rq->lock is not quite as disastrous as it usually is
> > +		 * because !cpu_active at this point, which means load-balance
> > +		 * will not interfere.
> > +		 */
> > +		lockdep_unpin_lock(&rq->lock);
> > +		raw_spin_unlock(&rq->lock);
> > +		raw_spin_lock(&next->pi_lock);
> > +		raw_spin_lock(&rq->lock);
> > +		lockdep_pin_lock(&rq->lock);
> > +		if (!(task_rq(next) == rq && task_on_rq_queued(next))) {
> > +			lockdep_unpin_lock(&rq->lock);
> > +			raw_spin_unlock(&rq->lock);
> Dropping rq->lock here means we will continue the loop without the
> rq->lock, right? But we do have a lockdep_pin_lock(&rq->lock) in the
> beginning of every iteration. So can we release rq->lock here?

Good catch! There is no need to lockdep_unpin and unlock rq->lock I think.

Regards,
Wanpeng Li

> Regards,
> Boqun
>
> > +			raw_spin_unlock(&next->pi_lock);
> > +			continue;
> > +		}
> > +
> >   		/* Find suitable destination for @next, with force if needed. */
> >   		dest_cpu = select_fallback_rq(dead_rq->cpu, next);
> >   
> > @@ -5196,6 +5217,7 @@ static void migrate_tasks(struct rq *dead_rq)
> >   			rq = dead_rq;
> >   			raw_spin_lock(&rq->lock);
> >   		}
> > +		raw_spin_unlock(&next->pi_lock);
> >   	}
> >   
> >   	rq->stop = stop;
> > --
> > 1.7.1
> >
> > --
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