linux-kernel - [PATCH 01/11] x86 rwsem: stay on fast path when count>0 in __up

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Date:	Mon, 24 May 2010 13:31:11 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: [PATCH 01/11] x86 rwsem: stay on fast path when count>0 in __up_write()

When count>0 there is no need to take the call_rwsem_wake path.
If we did take that path, it would just return without doing anything
due to the active count not being zero.

Signed-off-by: Michel Lespinasse <walken@...gle.com>
Acked-by: David Howells <dhowells@...hat.com>
---
 arch/x86/include/asm/rwsem.h |    5 ++---
 1 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index 606ede1..3fd1ed3 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -216,9 +216,8 @@ static inline void __up_write(struct rw_semaphore *sem)
 	rwsem_count_t tmp;
 	asm volatile("# beginning __up_write\n\t"
 		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
-		     /* tries to transition
-			0xffff0001 -> 0x00000000 */
-		     "  jz       1f\n"
+		     /* substracts 0xffff0001, returns the old value */
+		     "  jns        1f\n\t"
 		     "  call call_rwsem_wake\n"
 		     "1:\n\t"
 		     "# ending __up_write\n"
-- 
1.7.0.1

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