On Tue, 2013-06-11 at 10:02 -0700, Paul E. McKenney wrote:
> +#ifdef CONFIG_TICKET_LOCK_QUEUED
> +
> +#define __TKT_SPIN_INC 2
> +bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets inc);
> +
> +#else /* #ifdef CONFIG_TICKET_LOCK_QUEUED */
> +
> +#define __TKT_SPIN_INC 1
> +static inline bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets
> inc)
> +{
> + return false;
> +}
> +
> +#endif /* #else #ifdef CONFIG_TICKET_LOCK_QUEUED */
> +
> /*
> * Ticket locks are conceptually two parts, one indicating the current head
> of
> * the queue, and the other indicating the current tail. The lock is acquired
> @@ -49,17 +64,15 @@
> */
> static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
> {
> - register struct __raw_tickets inc = { .tail = 1 };
> + register struct __raw_tickets inc = { .tail = __TKT_SPIN_INC };
>
> inc = xadd(&lock->tickets, inc);
> -
> for (;;) {
> - if (inc.head == inc.tail)
> + if (inc.head == inc.tail || tkt_spin_pass(lock, inc))
> break;
> - cpu_relax();
Overheating the CPU are we ;-)
Keeping the cpu_relax() doesn't hurt, even when TICKET_LOCK_QUEUE is
enabled. As the only latency to worry about is when tkt_spin_pass()
returns true, where it breaks out of the loop anyway.
But if you really don't want the double call to cpu_relax(), we can
probably remove the cpu_relax from tkt_spin_pass() and keep this one, or
in the above tkt_spin_pass() where TICK_LOCK_QUEUED is not set, we can
do:
static inline bool tkt_spin_pass(arch_spinlock_t *ap, struct
__raw_tickets inc)
{
cpu_relax();
return false;
}
Honesty, I would say remove it from tkt_spin_pass() when returning
false.
-- Steve
> inc.head = ACCESS_ONCE(lock->tickets.head);
> }
> - barrier(); /* make sure nothing creeps before the lock is
> taken */
> + barrier(); /* Make sure nothing creeps in before the lock is taken. */
> }
>
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