Messages in this thread | | | Date | Fri, 31 Jan 2014 11:17:29 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation |
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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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