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

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Date:	Fri, 31 Jan 2014 11:17:29 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	George Spelvin <linux@...izon.com>
Cc:	Waiman.Long@...com, akpm@...ux-foundation.org, andi@...stfloor.org,
	arnd@...db.de, aswin@...com, hpa@...or.com,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com, paulmck@...ux.vnet.ibm.com,
	raghavendra.kt@...ux.vnet.ibm.com, riel@...hat.com,
	rostedt@...dmis.org, scott.norton@...com, tglx@...utronix.de,
	tim.c.chen@...ux.intel.com, torvalds@...ux-foundation.org,
	walken@...gle.com, x86@...nel.org
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock
 implementation

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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