x86, rwsem: Minor cleanups

Clarified few comments and made initialization of %edx/%rdx more uniform
accross __down_write_nested, __up_read and __up_write functions.

Signed-off-by: Michel Lespinasse <walken@google.com>
LKML-Reference: <201007202219.o6KMJkiA021048@imap1.linux-foundation.org>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Mike Waychison <mikew@google.com>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

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"
@@ -156,19 +156,17 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
 {
 	rwsem_count_t tmp;
-
-	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"
 		     "  call call_rwsem_down_write_failed\n"
 		     "1:\n"
 		     "# ending down_write"
 		     : "+m" (sem->count), "=d" (tmp)
-		     : "a" (sem), "1" (tmp)
+		     : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS)
 		     : "memory", "cc");
 }
 
@@ -195,16 +193,16 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
  */
 static inline void __up_read(struct rw_semaphore *sem)
 {
-	rwsem_count_t tmp = -RWSEM_ACTIVE_READ_BIAS;
+	rwsem_count_t tmp;
 	asm volatile("# beginning __up_read\n\t"
 		     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)
-		     : "a" (sem), "1" (tmp)
+		     : "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS)
 		     : "memory", "cc");
 }
 
@@ -218,7 +216,7 @@ static inline void __up_write(struct rw_semaphore *sem)
 		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
 		     /* subtracts 0xffff0001, 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\t"
 		     "# ending __up_write\n"
 		     : "+m" (sem->count), "=d" (tmp)