Re: [PATCH v2 4/4] futex: Avoid taking hb lock if nothing to wakeup

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Dec 03, 2013 at 01:45:27AM -0800, Davidlohr Bueso wrote:
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -82,10 +82,12 @@
>   * The waker side modifies the user space value of the futex and calls
>   * futex_wake(). It computes the hash bucket and acquires the hash
>   * bucket lock. Then it looks for waiters on that futex in the hash
> - * bucket and wakes them.
> - *
> - * Note that the spin_lock serializes waiters and wakers, so that the
> - * following scenario is avoided:
> + * bucket and wakes them.

Why not let this be the start of a new paragraph?

> In scenarios where wakeups are called and no
> + * tasks are blocked on a futex, taking the hb spinlock can be avoided
> + * and simply return. In order for this optimization to work, ordering
> + * guarantees must exist so that the waiter being added to the list is
> + * acknowledged when the list is concurrently being checked by the waker,
> + * avoiding scenarios like the following:
>   *
>   * CPU 0                               CPU 1
>   * val = *futex;
> @@ -106,24 +108,40 @@
>   * This would cause the waiter on CPU 0 to wait forever because it
>   * missed the transition of the user space value from val to newval
>   * and the waker did not find the waiter in the hash bucket queue.
> + * The correct serialization ensures that a waiter either observes
> + * the changed user space value before blocking or is woken by a
> + * concurrent waker:
>   *
>   * CPU 0                               CPU 1
>   * val = *futex;
>   * sys_futex(WAIT, futex, val);
>   *   futex_wait(futex, val);
> + *
> + *   mb(); <-- paired with ------
> + *                              |
> + *   lock(hash_bucket(futex));  |
> + *                              |
> + *   uval = *futex;             |
> + *                              |        *futex = newval;
> + *                              |        sys_futex(WAKE, futex);
> + *                              |          futex_wake(futex);
> + *                              |
> + *                              -------->   mb();
>   *   if (uval == val)
> + *     queue();
>   *     unlock(hash_bucket(futex));
> + *     schedule();                         if (!queue_empty())
> + *                                           lock(hash_bucket(futex));
> + *                                           wake_waiters(futex);
> + *                                           unlock(hash_bucket(futex));
> + *
> + * The length of the list is tracked with atomic ops (hb->waiters),
> + * providing the necessary memory barriers for the waiters. For the
> + * waker side, however, we rely on get_futex_key_refs(), using either
> + * ihold() or the atomic_inc(), for shared futexes. The former provides
> + * a full mb on all architectures. For architectures that do not have an
> + * implicit barrier in atomic_inc/dec, we explicitly add it - please
> + * refer to futex_get_mm() and hb_waiters_inc/dec().
>   */

This comment actually confuses me :/

It isn't at all explained what purpose the memory barriers serve.