Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

Davidlohr Bueso
2013-11-23T03:19:40Z

On Fri, 2013-11-22 at 17:25 -0800, Linus Torvalds wrote:
> On Fri, Nov 22, 2013 at 4:56 PM, Davidlohr Bueso <davidlohr@hp.com> wrote:
> > In futex_wake() there is clearly no point in taking the hb->lock if
> > we know beforehand that there are no tasks to be woken. This comes
> > at the smaller cost of doing some atomic operations to keep track of
> > the list's size.
>
> Hmm. Why? Afaik, you only care about "empty or not". And if you don't
> need the serialization from locking, then afaik you can just do a
> "plist_head_empty()" without holding the lock.

I remember this being the original approach, but after noticing some
strange behavior we quickly decided it wasn't the path. And sure enough,
I just double checked and tried the patch without atomic ops and can see
things being off: one of the futextest performance cases is stuck
blocked on a futex and I couldn't reboot the machine either -- nothing
apparent in dmesg, just not 100% functional. The thing is, we can only
avoid taking the lock only if nobody else is trying to add itself to the
list.

Thanks,
Davidlohr