Re: [PATCH] sched: Fix data-race in wakeup

[Valentin Schneider <valentin.schneider@arm.com>]

On 18/11/20 08:05, Peter Zijlstra wrote:
> 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.
>

Gotcha

> 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?

Yep, thanks!