Messages in this thread | | | Date | Thu, 4 Jun 2009 22:49:58 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] cpuhotplug: introduce try_get_online_cpus() take 2 |
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On 06/04, Lai Jiangshan wrote: > > - Lockless for get_online_cpus()'s fast path > - Introduce try_get_online_cpus()
I think this can work...
> @@ -50,10 +57,20 @@ void get_online_cpus(void) > might_sleep(); > if (cpu_hotplug.active_writer == current) > return; > - mutex_lock(&cpu_hotplug.lock); > - cpu_hotplug.refcount++; > - mutex_unlock(&cpu_hotplug.lock); > > + if (unlikely(!atomic_inc_not_zero(&cpu_hotplug.refcount))) { > + DEFINE_WAIT(wait); > + > + for (;;) { > + prepare_to_wait(&cpu_hotplug.sleeping_readers, &wait, > + TASK_UNINTERRUPTIBLE); > + if (atomic_inc_not_zero(&cpu_hotplug.refcount)) > + break; > + schedule(); > + } > + > + finish_wait(&cpu_hotplug.sleeping_readers, &wait); > + } > }
Looks like the code above can be replaced with
wait_event(atomic_inc_not_zero(&cpu_hotplug.refcount));
> static void cpu_hotplug_done(void) > { > cpu_hotplug.active_writer = NULL; > - mutex_unlock(&cpu_hotplug.lock); > + atomic_inc(&cpu_hotplug.refcount); > + > + if (waitqueue_active(&cpu_hotplug.sleeping_readers)) > + wake_up(&cpu_hotplug.sleeping_readers); > }
This looks racy.
Suppose that the new reader comes right before atomic_inc(). The first inc_not_zero() fails, the readear does prepare_to_wait(), the 2nd inc_not_zero() fails too.
cpu_hotplug_done() does atomic_inc().
What guarantees we must see waitqueue_active() == T?
I think cpu_hotplug_done() should do unconditional wake_up(). This path is slow anyway, "if (waitqueue_active())" does not buy too much. In this case .sleeping_readers->lock closes the race.
Unless I missed something, of course.
Minor, but I'd suggest to use wake_up_all(). This does not make any difference because we do not have WQ_FLAG_EXCLUSIVE waiters, but imho looks a bit cleaner.
Hmm. It seems to me that cpu_hotplug_done() needs mb__before_atomic_inc() before atomic_inc. Otherwise, "active_writer = NULL" can be re-ordered with atomic_inc(). If the new reader does get_online_cpus() + put_online_cpus() quicky, it can see active_writer != NULL.
Oleg.
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