[Xen-devel] Re: [PATCH RFC V5 00/11] Paravirtualized ticketlocks

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Wed, 2011-10-12 at 17:51 -0700, Jeremy Fitzhardinge wrote:
> 
> This is is all unnecessary complication if you're not using PV ticket
> locks, it also uses the jump-label machinery to use the standard
> "add"-based unlock in the non-PV case.
> 
>         if (TICKET_SLOWPATH_FLAG &&
>             unlikely(static_branch(&paravirt_ticketlocks_enabled))) {
>                 arch_spinlock_t prev;
> 
>                 prev = *lock;
>                 add_smp(&lock->tickets.head, TICKET_LOCK_INC);
> 
>                 /* add_smp() is a full mb() */
> 
>                 if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
>                         __ticket_unlock_slowpath(lock, prev);
>         } else
>                 __add(&lock->tickets.head, TICKET_LOCK_INC, 
> UNLOCK_LOCK_PREFIX); 

Not that I mind the jump_label usage, but didn't paravirt have an
existing alternative() thingy to do things like this? Or is the
alternative() stuff not flexible enough to express this?

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