From: Heiko Carstens <heiko.carstens@de.ibm.com>
Date: Mon, 23 Sep 2013 10:59:56 +0000 (+0200)
Subject: lockref: use arch_mutex_cpu_relax() in CMPXCHG_LOOP()
X-Git-Tag: v3.12-rc3~14^2~1
X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=commitdiff_plain;h=491f6f8e5fd9a57aaf03b6d6e3e153f1c27d8a46;p=linux.git

lockref: use arch_mutex_cpu_relax() in CMPXCHG_LOOP()

Make use of arch_mutex_cpu_relax() so architectures can override the
default cpu_relax() semantics.
This is especially useful for s390, where cpu_relax() means that we
yield() the current (virtual) cpu and therefore is very expensive,
and would contradict the whole purpose of the lockless cmpxchg loop.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---

diff --git a/lib/lockref.c b/lib/lockref.c
index e294ae445c9a..6f9d434c1521 100644
--- a/lib/lockref.c
+++ b/lib/lockref.c
@@ -11,6 +11,14 @@
 # define cmpxchg64_relaxed cmpxchg64
 #endif
 
+/*
+ * Allow architectures to override the default cpu_relax() within CMPXCHG_LOOP.
+ * This is useful for architectures with an expensive cpu_relax().
+ */
+#ifndef arch_mutex_cpu_relax
+# define arch_mutex_cpu_relax() cpu_relax()
+#endif
+
 /*
  * Note that the "cmpxchg()" reloads the "old" value for the
  * failure case.
@@ -28,7 +36,7 @@
 		if (likely(old.lock_count == prev.lock_count)) {		\
 			SUCCESS;						\
 		}								\
-		cpu_relax();							\
+		arch_mutex_cpu_relax();						\
 	}									\
 } while (0)