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)"? The comments are great, just try to understand your meaning here ;-) Regards, Boqun > + */ > + > /** > * try_to_wake_up - wake up a thread > * @p: the thread to be awakened
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