Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

[Peter Zijlstra <peterz@infradead.org>]

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.