On Fri, Nov 20, 2015 at 10:08:50PM +0800, Boqun Feng wrote: > Hi Peter, > > On Fri, Nov 20, 2015 at 11:02:30AM +0100, Peter Zijlstra wrote: > [snip] > > + * BLOCKING -- aka. SLEEP + WAKEUP > > + * > > + * For blocking we (obviously) need to provide the same guarantee as for > > + * migration. However the means are completely different as there is no > > lock > > + * chain to provide order. Instead we do: > > + * > > + * 1) smp_store_release(X->on_cpu, 0) > > + * 2) smp_cond_acquire(!X->on_cpu) > > + * > > + * Example: > > + * > > + * CPU0 (schedule) CPU1 (try_to_wake_up) CPU2 (schedule) > > + * > > + * LOCK rq(0)->lock LOCK X->pi_lock > > + * dequeue X > > + * sched-out X > > + * smp_store_release(X->on_cpu, 0); > > + * > > + * smp_cond_acquire(!X->on_cpu); > > + * X->state = WAKING > > + * set_task_cpu(X,2) > > + * > > + * LOCK rq(2)->lock > > + * enqueue X > > + * X->state = RUNNING > > + * UNLOCK rq(2)->lock > > + * > > + * LOCK rq(2)->lock // orders > > against CPU1 > > + * sched-out Z > > + * sched-in X > > + * UNLOCK rq(1)->lock > > + * > > + * UNLOCK X->pi_lock > > + * UNLOCK rq(0)->lock > > + * > > + * > > + * However; for wakeups there is a second guarantee we must provide, > > namely we > > + * must observe the state that lead to our wakeup. That is, not only must > > our > > + * task observe its own prior state, it must also observe the stores prior > > to > > + * its wakeup. > > + * > > + * This means that any means of doing remote wakeups must order the CPU > > doing > > + * the wakeup against the CPU the task is going to end up running on. This, > > + * however, is already required for the regular Program-Order guarantee > > above, > > + * since the waking CPU is the one issueing the ACQUIRE (2). > > + * > > Hope I'm the only one who got confused about the "2" in "ACQUIRE (2)", > what does that refer? "2) smp_cond_acquire(!X->on_cpu)"?
Yes, exactly that. Would an unadorned 2 be clearer? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
