On Thu, 2013-12-19 at 19:13 -0800, Linus Torvalds wrote: > On Thu, Dec 19, 2013 at 6:22 PM, Davidlohr Bueso <davidl...@hp.com> 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 -- 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/
