Commit e1d7c760 authored by Uros Bizjak's avatar Uros Bizjak Committed by Andrew Morton
Browse files

bitops: use try_cmpxchg in set_mask_bits and bit_clear_unless 

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.

Link: https://lkml.kernel.org/r/20220822143851.3290-1-ubizjak@gmail.com


Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 21490eff

include/linux/bitops.h

+6 −5
Original line number Diff line number Diff line
@@ -328,10 +328,10 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr, 
	const typeof(*(ptr)) mask__ = (mask), bits__ = (bits);	\

	typeof(*(ptr)) old__, new__;				\

								\

	do {							\

	old__ = READ_ONCE(*(ptr));				\

	do {							\

		new__ = (old__ & ~mask__) | bits__;		\

	} while (cmpxchg(ptr, old__, new__) != old__);		\

	} while (!try_cmpxchg(ptr, &old__, new__));		\

								\

	old__;							\

})
@@ -343,11 +343,12 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr, 
	const typeof(*(ptr)) clear__ = (clear), test__ = (test);\

	typeof(*(ptr)) old__, new__;				\

								\

	do {							\

	old__ = READ_ONCE(*(ptr));				\

	do {							\

		if (old__ & test__)				\

			break;					\

		new__ = old__ & ~clear__;			\

	} while (!(old__ & test__) &&				\

		 cmpxchg(ptr, old__, new__) != old__);		\

	} while (!try_cmpxchg(ptr, &old__, new__));		\

								\

	!(old__ & test__);					\

})