From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock
 implementation
Date: Tue, 18 Feb 2014 22:34:00 +0100
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To: Waiman Long <waiman.long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>, linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>, Andrew Morton <akpm@linux-foundation.org>, Michel Lespinasse <walken@google.com>, Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Linus Torvalds <torvalds@linux-foundation.org>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, George Spelvin <linux@horizon.com>, Tim Chen <tim.c.chen@linux.intel.com>, Daniel J Blueman <daniel@numascale.com>, Alexander Fyodorov <halcy@yandex.ru>, Aswin Chandramouleeswaran <aswin@hp.com>, Scott J Norton <scott.norton@hp.com>, Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>

On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:
> The #ifdef is harder to take away here. The point is that doing a 32-bit
> exchange may accidentally steal the lock with the additional code to handle
> that. Doing a 16-bit exchange, on the other hand, will never steal the lock
> and so don't need the extra handling code. I could construct a function with
> different return values to handle the different cases if you think it will
> make the code easier to read.

Does it really pay to use xchg() with all those fixup cases? Why not
have a single cmpxchg() loop that does just the exact atomic op you
want?