+ bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless.patch added to mm-nonmm-unstable branch

+ bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: bitops: use try_cmpxchg in set_mask_bits and bit_clear_unless
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Uros Bizjak <ubizjak@gmail.com>
Subject: bitops: use try_cmpxchg in set_mask_bits and bit_clear_unless
Date: Mon, 22 Aug 2022 16:38:51 +0200

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: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/bitops.h |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/include/linux/bitops.h~bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless
+++ a/include/linux/bitops.h
@@ -327,10 +327,10 @@ static __always_inline void __assign_bit
 	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
 	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__);					\
 })
_

Patches currently in -mm which might be from ubizjak@gmail.com are

llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch
epoll-use-try_cmpxchg-in-list_add_tail_lockless.patch
buffer-use-try_cmpxchg-in-discard_buffer.patch
aio-use-atomic_try_cmpxchg-in-__get_reqs_available.patch
iversion-use-atomic64_try_cmpxchg.patch
bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless.patch