Date: Fri, 31 Jan 2014 11:17:29 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: George Spelvin <linux@horizon.com>
Cc: Waiman.Long@hp.com, akpm@linux-foundation.org, andi@firstfloor.org,
        arnd@arndb.de, aswin@hp.com, hpa@zytor.com, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, mingo@redhat.com,
        paulmck@linux.vnet.ibm.com, raghavendra.kt@linux.vnet.ibm.com,
        riel@redhat.com, rostedt@goodmis.org, scott.norton@hp.com,
        tglx@linutronix.de, tim.c.chen@linux.intel.com,
        torvalds@linux-foundation.org, walken@google.com, x86@kernel.org
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock
 implementation
Message-ID: <20140131101729.GA8874@twins.programming.kicks-ass.net>
References: <20140131092616.GC5126@laptop.programming.kicks-ass.net>
 <20140131100348.8867.qmail@science.horizon.com>
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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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