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

Jason Low
2013-11-23T03:03:44Z

On Fri, Nov 22, 2013 at 5:25 PM, Linus Torvalds
<torvalds@linux-foundation.org> 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.

Hi Linus,

In this patch, since we do the atomic increments before holding the
hb->lock during situations where we may potentially add a task to the
list, then we would only skip attempting the wake up if no other
thread has already held the hb->lock and is in the process of adding a
task to the list, in addition to the list being empty. Would this be
enough to protect it from any race condition?

Thanks,
Jason