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

Peter Zijlstra
2013-12-10T17:34:28Z

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.