From 8f8dcb3f7fe4febbfa96e64d4ad47de958c5cc34 Mon Sep 17 00:00:00 2001
From: Matt Turner <mattst88@gmail.com>
Date: Tue, 4 Aug 2015 14:35:05 -0700
Subject: [PATCH] alpha: select CONFIG_ARCH_USE_CMPXCHG_LOCKREF.

On Alpha we have spinlocks that are 32b in size and an efficient
cmpxchg64 implementation, so we qualify to make use of cmpxchg backed
lockrefs. Select the ARCH_USE_CMPXCHG_LOCKREF Kconfig symbol and provide
a trivial implementation of arch_spin_value_unlocked to satisfy the
lockref code.

Using Linus' simple testcase from
http://article.gmane.org/gmane.linux.file-systems/77466 on a dual CPU
ES47 system I see around an 8% gain:

    N       Min       Max    Median       Avg      Stddev
x  30   6194580   6295654   6272504   6272514   17694.232
+  30   6731164   6786334   6767982   6764274   13738.863
Difference at 95.0% confidence
        491760 +/- 8188.17
        7.83992% +/- 0.130541%
        (Student's t, pooled s = 15840.5)

Signed-off-by: Matt Turner <mattst88@gmail.com>
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