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@xxxxxxxx> 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(¶virt_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
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