On Tue, Nov 17, 2020 at 10:46:21AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 10:29:36AM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 17, 2020 at 09:15:46AM +0000, Will Deacon wrote:
> > > On Tue, Nov 17, 2020 at 09:30:16AM +0100, Peter Zijlstra wrote:
> > > > /* Unserialized, strictly 'current' */
> > > >
> > > > + /*
> > > > + * p->in_iowait = 1; 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
> > > > = X;
> > > > + *
> > > > + * Guarantees all stores of 'current' are visible before
> > > > + * ->sched_remote_wakeup gets used.
> > >
> > > I'm still not sure this is particularly clear -- don't we want to
> > > highlight
> > > that the store of p->on_rq is unordered wrt the update to
> > > p->sched_contributes_to_load() in deactivate_task()?
>
> How's this then? It still doesn't explicitly call out the specific race,
> but does mention the more fundamental issue that wakelist queueing
> doesn't respect the regular rules anymore.
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -775,7 +775,6 @@ struct task_struct {
> unsigned sched_reset_on_fork:1;
> unsigned sched_contributes_to_load:1;
> unsigned sched_migrated:1;
> - unsigned sched_remote_wakeup:1;
> #ifdef CONFIG_PSI
> unsigned sched_psi_wake_requeue:1;
> #endif
> @@ -785,6 +784,21 @@ struct task_struct {
>
> /* Unserialized, strictly 'current' */
>
> + /*
> + * 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.
> + */
> + unsigned sched_remote_wakeup:1;
Much better, thanks!
Will
