On Fri, Nov 20, 2015 at 03:18:12PM +0100, Peter Zijlstra wrote: > 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?
How about "the one issueing the ACQUIRE (smp_cond_acquire)"?
signature.asc
Description: PGP signature
