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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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.

It makes gcc go into paroxysms of trickiness:

 4a8:   80 3f fe                cmpb   $0xfe,(%rdi)
 4ab:   19 f6                   sbb    %esi,%esi
 4ad:   66 81 e6 00 01          and    $0x100,%si
 4b2:   66 81 ee fe 00          sub    $0xfe,%si
 4b7:   f0 66 0f c1 37          lock xadd %si,(%rdi)

...which is pretty neat, actually.

    J