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From: Michel Lespinasse on 14 May 2010 08:50
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(a)google.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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