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>

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index bf9e9d3b379218b74aa2bf6506d62f7978f71abd..f515a4dbf7a0621e902fc488ef0108ac34d60b4c 100644 (file)
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -3,6 +3,7 @@ config ALPHA
        default y
        select ARCH_MIGHT_HAVE_PC_PARPORT
        select ARCH_MIGHT_HAVE_PC_SERIO
+       select ARCH_USE_CMPXCHG_LOCKREF
        select HAVE_AOUT
        select HAVE_IDE
        select HAVE_OPROFILE

diff --git a/arch/alpha/include/asm/spinlock.h b/arch/alpha/include/asm/spinlock.h
index 37b570d01202e4ee62d5dcc11af345f1805e81a0..fed9c6f44c191295d36f4ab8a9a9352883a1c5b1 100644 (file)
--- a/arch/alpha/include/asm/spinlock.h
+++ b/arch/alpha/include/asm/spinlock.h
@@ -16,6 +16,11 @@
 #define arch_spin_unlock_wait(x) \
                do { cpu_relax(); } while ((x)->lock)
 
+static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
+{
+        return lock.lock == 0;
+}
+
 static inline void arch_spin_unlock(arch_spinlock_t * lock)
 {
        mb();