[tip: locking/core] futex: Use atomic64_try_cmpxchg_relaxed() in get_inode_sequence_number()

[tip-bot2 for Uros Bizjak]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     87347f148061b48c3495fb61dcbad384760da9cf
Gitweb:        https://git.kernel.org/tip/87347f148061b48c3495fb61dcbad384760da9cf
Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate:    Thu, 10 Oct 2024 09:10:05 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 17 Oct 2024 22:02:27 +02:00

futex: Use atomic64_try_cmpxchg_relaxed() in get_inode_sequence_number()

Optimize get_inode_sequence_number() to use simpler and faster:

  !atomic64_try_cmpxchg_relaxed(*ptr, &old, new)

instead of:

  atomic64_cmpxchg relaxed(*ptr, old, new) != old

The x86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg. The generated
code improves from:

 3da:	31 c0                	xor    %eax,%eax
 3dc:	f0 48 0f b1 8a 38 01 	lock cmpxchg %rcx,0x138(%rdx)
 3e3:	00 00
 3e5:	48 85 c0             	test   %rax,%rax
 3e8:	48 0f 44 c1          	cmove  %rcx,%rax

to:

 3da:	31 c0                	xor    %eax,%eax
 3dc:	f0 48 0f b1 8a 38 01 	lock cmpxchg %rcx,0x138(%rdx)
 3e3:	00 00
 3e5:	48 0f 44 c1          	cmove  %rcx,%rax

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: André Almeida <andrealmeid@xxxxxxxxxx>
Link: https://lore.kernel.org/all/20241010071023.21913-2-ubizjak@xxxxxxxxx

---
 kernel/futex/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 3146730..692912b 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -185,8 +185,8 @@ static u64 get_inode_sequence_number(struct inode *inode)
 		if (WARN_ON_ONCE(!new))
 			continue;
 
-		old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
-		if (old)
+		old = 0;
+		if (!atomic64_try_cmpxchg_relaxed(&inode->i_sequence, &old, new))
 			return old;
 		return new;
 	}