Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

[Stephan Diestelhorst <stephan.diestelhorst@amd.com>]

On Wednesday 28 September 2011 19:50:08 Jeremy Fitzhardinge wrote:
> On 09/28/2011 10:24 AM, H. Peter Anvin wrote:
> > On 09/28/2011 10:22 AM, Linus Torvalds wrote:
> >> On Wed, Sep 28, 2011 at 9:47 AM, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
> >>> Could do something like:
> >>>
> >>>        if (ticket->head >= 254)
> >>>                prev = xadd(&ticket->head_tail, 0xff02);
> >>>        else
> >>>                prev = xadd(&ticket->head_tail, 0x0002);
> >>>
> >>> to compensate for the overflow.
> >> Oh wow. You havge an even more twisted mind than I do.
> >>
> >> I guess that will work, exactly because we control "head" and thus can
> >> know about the overflow in the low byte. But boy is that ugly ;)
> >>
> >> But at least you wouldn't need to do the loop with cmpxchg. So it's
> >> twisted and ugly, but migth be practical.
> >>
> > I suspect it should be coded as -254 in order to use a short immediate
> > if that is even possible...
> 
> I'm about to test:
> 
> static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
> {
> 	if (TICKET_SLOWPATH_FLAG && unlikely(arch_static_branch(&paravirt_ticketlocks_enabled))) {
> 		arch_spinlock_t prev;
> 		__ticketpair_t inc = TICKET_LOCK_INC;
> 
> 		if (lock->tickets.head >= (1 << TICKET_SHIFT) - TICKET_LOCK_INC)
> 			inc += -1 << TICKET_SHIFT;
> 
> 		prev.head_tail = xadd(&lock->head_tail, inc);
> 
> 		if (prev.tickets.tail & TICKET_SLOWPATH_FLAG)
> 			__ticket_unlock_slowpath(lock, prev);
> 	} else
> 		__ticket_unlock_release(lock);
> }
> 
> Which, frankly, is not something I particularly want to put my name to.

I must have missed the part when this turned into the propose-the-
craziest-way-that-this-still-works.contest :)

What is wrong with converting the original addb into a lock addb? The
crazy wrap around tricks add a conditional and lots of headache. The
lock addb/w is clean. We are paying an atomic in both cases, so I just
don't see the benefit of the second solution.

Stephan
-- 
Stephan Diestelhorst, AMD Operating System Research Center
stephan.diestelhorst@amd.com, Tel. +49 (0)351 448 356 719

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