[Xen-devel] Re: [PATCH 13/14] x86/ticketlock: add slowpath logic

[Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>]

> On Mon, Jan 24, 2011 at 01:56:53PM -0800, Jeremy Fitzhardinge wrote:

For some reason, I seem to be missing emails from your id/domain and hence had
missed this completely!

> >   * bits.  However, we need to be careful about this because someone
> >   * may just be entering as we leave, and enter the slowpath.
> >   */
> > -void __ticket_unlock_release_slowpath(struct arch_spinlock *lock)
> > +void __ticket_unlock_slowpath(struct arch_spinlock *lock)
> >  {
> >     struct arch_spinlock old, new;
> >  
> >     BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
> >  
> >     old = ACCESS_ONCE(*lock);
> > -
> >     new = old;
> > -   new.tickets.head += TICKET_LOCK_INC;
> >  
> >     /* Clear the slowpath flag */
> >     new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
> > +   if (new.tickets.head == new.tickets.tail)
> > +           cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
> >  
> > -   /*
> > -    * If there's currently people waiting or someone snuck in
> > -    * since we read the lock above, then do a normal unlock and
> > -    * kick.  If we managed to unlock with no queued waiters, then
> > -    * we can clear the slowpath flag.
> > -    */
> > -   if (new.tickets.head != new.tickets.tail ||
> > -       cmpxchg(&lock->head_tail,
> > -               old.head_tail, new.head_tail) != old.head_tail) {
> > -           /* still people waiting */
> > -           __ticket_unlock_release(lock);
> > -   }
> > -
> > +   /* Wake up an appropriate waiter */
> >     __ticket_unlock_kick(lock, new.tickets.head);
> 
> Does the __ticket_unlock_kick need to be unconditional?

I recall having tried optimizing it to be conditional, something along these
lines:

        if (new.ticket.head == new.tickets.tail) {
                cmpxchg();      
        } else {
                __ticket_unlock_kick(lock, new.tickets.head);
        }

but it didn't work for some reason. I left the call unconditional as was the
case previously based on that experiment.

- vatsa

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