On Fri, Jan 31, 2014 at 05:03:48AM -0500, George Spelvin wrote:
> How about getting rid of that TICKET_MSB mess and doing something like:
>
> #define TICKET_MASK 0xFFFF
>
> static inline void ticket_spin_unlock(atomic_t *tickets)
> {
> u32 t = *tickets;
>
> smp_mb__before_atomic_inc();
>
> /* Increment the low 16 bits without affecting the upper. */
> if (unlikely((~t & TICKET_MASK) == 0))
> atomic_add(-(atomic_t)TICKET_MASK, tickets);
> else
> atomic_inc(tickets);
> }
>
> That also allows up to 2^16 waiters, up from 2^15.
> (Minus one on both cases, if you want to be fussy.)
Ah indeed. That'll work. That said, any arch that can single-copy
address shorts can probably do better than this generic atomic_t thing.
My main point was that we should seriously look at a ticket lock instead
of the MCS one, because while the MCS has better contention behaviour,
we shouldn't optimize locks for the worst contention.
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