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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
