[PATCH] sched: Fix data-race in wakeup

[Valentin Schneider]

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;