[PATCH] bitops: Use try_cmpxchg in set_mask_bits and bit_clear_unless

[Uros Bizjak]

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
set_mask_bits and bit_clear_unless. x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails, enabling further code simplifications.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
 include/linux/bitops.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index cf9bf65039f2..7d1a3b01d1b0 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -327,10 +327,10 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr,
 	const typeof(*(ptr)) mask__ = (mask), bits__ = (bits);	\
 	typeof(*(ptr)) old__, new__;				\
 								\
+	old__ = READ_ONCE(*(ptr));				\
 	do {							\
-		old__ = READ_ONCE(*(ptr));			\
 		new__ = (old__ & ~mask__) | bits__;		\
-	} while (cmpxchg(ptr, old__, new__) != old__);		\
+	} while (!try_cmpxchg(ptr, &old__, new__));		\
 								\
 	old__;							\
 })
@@ -342,11 +342,12 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr,
 	const typeof(*(ptr)) clear__ = (clear), test__ = (test);\
 	typeof(*(ptr)) old__, new__;				\
 								\
+	old__ = READ_ONCE(*(ptr));				\
 	do {							\
-		old__ = READ_ONCE(*(ptr));			\
+		if (old__ & test__)				\
+			break;					\
 		new__ = old__ & ~clear__;			\
-	} while (!(old__ & test__) &&				\
-		 cmpxchg(ptr, old__, new__) != old__);		\
+	} while (!try_cmpxchg(ptr, &old__, new__));		\
 								\
 	!(old__ & test__);					\
 })
-- 
2.37.1