[PATCH] sched: Fix data-race in wakeup

[Peter Zijlstra]

On Tue, Nov 17, 2020 at 03:37:24PM +0000, Valentin Schneider wrote:

> >> +	/*
> >> +	 * This field must not be in the scheduler word above due to wakelist
> >> +	 * queueing no longer being serialized by p->on_cpu. However:
> >> +	 *
> >> +	 * p->XXX = X;			ttwu()
> >> +	 * schedule()			  if (p->on_rq && ..) // false
> >> +	 *   smp_mb__after_spinlock();	  if (smp_load_acquire(&p->on_cpu) && //true
> >> +	 *   deactivate_task()		      ttwu_queue_wakelist())
> >> +	 *     p->on_rq = 0;			p->sched_remote_wakeup = Y;
> >> +	 *
> >> +	 * guarantees all stores of 'current' are visible before
> >> +	 * ->sched_remote_wakeup gets used, so it can be in this word.
> >> +	 */
> >
> > Isn't the control dep between that ttwu() p->on_rq read and
> > p->sched_remote_wakeup write "sufficient"?
> 
> smp_acquire__after_ctrl_dep() that is, since we need
>   ->on_rq load => 'current' bits load + store

I don't think we need that extra barrier; after all, there will be a
complete schedule() between waking the task and it actually becoming
current.