Re: [PATCH v3 4/4] futex: Avoid taking hb lock if nothing to wakeup

[Davidlohr Bueso]

On Thu, 2013-12-19 at 19:13 -0800, Linus Torvalds wrote:
> On Thu, Dec 19, 2013 at 6:22 PM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> >
> > Ok so when I replied I was thinking about the plist really and not the
> > hb->lock ticket counter. Yeah, what you propose does make sense for
> > waiters. However in wake paths we have to decrement  the counter nwake
> > times (per each call to __unqueue_futex()), and if we rely on the
> > ticket, then we do it only once, in the unlock, so the counter doesn't
> > reflect the actual waitqueue size. Or am I missing something with ticket
> > spinlocks (I'm not very familiar with that code)?
> 
> So that's why I suggested checking the ticket lock _and_ the state of
> the node list.
> 
> The ticket lock state gives you the racy window before the entry has
> actually been added to the node list, and then after the lock has been
> released, the fact that the list isn't empty gives you the rest.

Yep, I'm starting to get around the idea now.

> I think we might have to order the two reads with an smp_rmb - making
> sure that we check the lock state first - but I think it should be
> otherwise pretty solid.

Yes, I don't see any guarantees otherwise.

> 
> And maybe there is some reason it doesn't work, but I'd really like to
> understand it (and I'd like to avoid the extra atomics for the waiters
> count if it at all can be avoided)

Other than the lock_ptr assignment I don't see much difference with what
you suggest. Since no wake paths use lock_ptr until after plist_del, and
we've already taken the lock way before then, it doesn't play a role in
what we're discussing.

I've put you suggestion (now corrected with spin_is_locked) and works
just as fine as with atomics. It's surviving my laptop and benchmarks on
a large system. I will wait to see if there are any concerns other might
have before sending another version of this patchset.

Thanks,
Davidlohr

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