On Tue, Dec 10, 2013 at 09:22:08AM -0800, Davidlohr Bueso wrote: > On Tue, 2013-12-10 at 17:57 +0100, Peter Zijlstra wrote: > > > @@ -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(). > > > */
> > It isn't at all explained what purpose the memory barriers serve. > > Why doesn't this explain it? Because you failed to explain what is ordered against what and what the resulting order guarantees. > "The correct serialization ensures that a waiter either observes > the changed user space value before blocking or is woken by a > concurrent waker." Or both. For the given case: X = Y = 0 w[X]=1 w[Y]=1 MB MB r[Y]=y r[X]=x x==1 && y==1 is a valid result. The only invalid result is both 0. But then we're still short of how we end up at that guarantee. > Perhaps adding an example? > plist_add() | uaddr = newval > smp_mb() | smp_mb() > verify uaddr | plist_head_empty() Except of course you don't actually use plist_add() and plist_head_empty() for anything much at all, only creating more confusion. Just add the waiters variable and explicitly mention what you're doing. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
