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    SubjectRe: [PATCH] cpuhotplug: introduce try_get_online_cpus() take 2
    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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