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

Waiman Long
2013-11-23T04:06:11Z

On 11/22/2013 08:25 PM, 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.
>
> NOTE!
>
> The "list_empty()" function is very much designed to work even without
> holding a lock (as long as the head itself exists reliably, of course)
> BUT you have to then guarantee yourself that your algorithm doesn't
> have any races wrt other CPU's adding an entry to the list at the same
> time. Not holding a lock obviously means that you are not serialized
> against that.. We've had problems with people doing
>
> if (!list_empty(waiters))
> wake_up_list(..)
>
> because they wouldn't wake people up who just got added.
>
> But considering that your atomic counter checking has the same lack of
> serialization, at least the plist_head_empty() check shouldn't be any
> worse than that counter thing.. And doesn't need any steenking atomic
> ops or a new counter field.
>
> Linus

Before the patch, a waker will wake up one or waiters who call
spin_lock() before the waker. If we just use plist_head_empty(), we will
miss waiters who have called spin_lock(), but have not yet got the lock
or inserted itself into the wait list. By adding atomic counter for
checking purpose, we again has a single point where any waiters who pass
that point (inc atomic counter) can be woken up by a waiter who check
the atomic counter after they inc it. This acts sort of like
serialization without using a lock.

-Longman