[PATCH] sched: Fix data-race in wakeup

[Peter Zijlstra]

On Tue, Nov 17, 2020 at 07:32:16PM +0000, Valentin Schneider wrote:
> 
> On 17/11/20 16:13, Peter Zijlstra wrote:
> > 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.
> 
> Apologies for the messy train of thought; what I was trying to say is that
> we have already the following, which AIUI is sufficient:
> 
> 	* p->XXX = X;			ttwu()
> 	* schedule()			  if (p->on_rq && ..) // false
> 	*   smp_mb__after_spinlock();	  smp_acquire__after_ctrl_dep();
> 	*   deactivate_task()		  ttwu_queue_wakelist()
> 	*     p->on_rq = 0;		    p->sched_remote_wakeup = Y;
> 

Ah, you meant the existing smp_acquire__after_ctrl_dep(). Yeah, that's
not required here either ;-)

The reason I had the ->on_cpu thing in there is because it shows we
violate the regular ->on_cpu handoff rules, not for the acquire.

The only ordering that matters on the RHS of that thing is the ->on_rq
load to p->sched_remote_wakeup store ctrl dep. That, combined with the
LHS, guarantees there is a strict order on the stores.

Makes sense?