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!
