linux-kernel - [PATCH 1/2] x86: optimize a bit __raw_read

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Message-ID: <497cd085.2283420a.5373.49e7@mx.google.com>
Date:	Sun, 25 Jan 2009 12:50:13 -0800 (PST)
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [PATCH 1/2] x86: optimize a bit __raw_read_trylock

The current version of __raw_read_trylock starts with decrementing the lock
and read its new value as a separate operation after that.

That makes 3 dereferences (read, write (after sub), read) whereas
a single atomic_dec_return does only two pointers dereferences (read, write).

Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 arch/x86/include/asm/spinlock.h |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 2bd6b11..139b424 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -267,8 +267,7 @@ static inline int __raw_read_trylock(raw_rwlock_t *lock)
 {
 	atomic_t *count = (atomic_t *)lock;
 
-	atomic_dec(count);
-	if (atomic_read(count) >= 0)
+	if (atomic_dec_return(count) >= 0)
 		return 1;
 	atomic_inc(count);
 	return 0;
-- 
1.6.1


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