linux-kernel - [PATCH 01/10] x86 rwsem: minor cleanups

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Date:	Fri, 14 May 2010 05:39:33 -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>,
	Michel Lespinasse <walken@...gle.com>
Subject: [PATCH 01/10] x86 rwsem: minor cleanups

The only functional change here is that __up_write won't call
call_rwsem_wake anymore if the new rwsem value is >0. This makes
no real difference since call_rwsem_wake would have noticed the
active count being nonzero and done nothing anyway.

Besides that, I clarified a few comments.

Feel free to drop this patch from the series if you don't like it;
nothing else depends on it.

Signed-off-by: Michel Lespinasse <walken@...gle.com>
---
 arch/x86/include/asm/rwsem.h |   17 ++++++++---------
 1 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index 606ede1..86e0473 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -118,7 +118,7 @@ static inline void __down_read(struct rw_semaphore *sem)
 {
 	asm volatile("# beginning down_read\n\t"
 		     LOCK_PREFIX _ASM_INC "(%1)\n\t"
-		     /* adds 0x00000001, returns the old value */
+		     /* adds 0x00000001 */
 		     "  jns        1f\n"
 		     "  call call_rwsem_down_read_failed\n"
 		     "1:\n\t"
@@ -160,7 +160,7 @@ static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
 	tmp = RWSEM_ACTIVE_WRITE_BIAS;
 	asm volatile("# beginning down_write\n\t"
 		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
-		     /* subtract 0x0000ffff, returns the old value */
+		     /* adds 0xffff0001, returns the old value */
 		     "  test      %1,%1\n\t"
 		     /* was the count 0 before? */
 		     "  jz        1f\n"
@@ -200,7 +200,7 @@ static inline void __up_read(struct rw_semaphore *sem)
 		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
 		     /* subtracts 1, returns the old value */
 		     "  jns        1f\n\t"
-		     "  call call_rwsem_wake\n"
+		     "  call call_rwsem_wake\n" /* expects old value in %edx */
 		     "1:\n"
 		     "# ending __up_read\n"
 		     : "+m" (sem->count), "=d" (tmp)
@@ -213,17 +213,16 @@ static inline void __up_read(struct rw_semaphore *sem)
  */
 static inline void __up_write(struct rw_semaphore *sem)
 {
-	rwsem_count_t tmp;
+	rwsem_count_t tmp = -RWSEM_ACTIVE_WRITE_BIAS;
 	asm volatile("# beginning __up_write\n\t"
 		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
-		     /* tries to transition
-			0xffff0001 -> 0x00000000 */
-		     "  jz       1f\n"
-		     "  call call_rwsem_wake\n"
+		     /* substracts 0xffff0001, returns the old value */
+		     "  jns        1f\n\t"
+		     "  call call_rwsem_wake\n" /* expects old value in %edx */
 		     "1:\n\t"
 		     "# ending __up_write\n"
 		     : "+m" (sem->count), "=d" (tmp)
-		     : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
+		     : "a" (sem), "1" (tmp)
 		     : "memory", "cc");
 }
 
-- 
1.7.0.1

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