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;