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SubjectRe: [PATCH] sched: Fix data-race in wakeup
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?

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