[PATCH] fs: Use try_cmpxchg() in start_dir_add()

[Uros Bizjak]

Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after CMPXCHG (and related
move instruction in front of CMPXCHG).

Note that the value from *ptr should be read using READ_ONCE() to
prevent the compiler from merging, refetching or reordering the read.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
---
 fs/dcache.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 60046ae23d51..336bdb4c4b1f 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2509,8 +2509,8 @@ static inline unsigned start_dir_add(struct inode *dir)
 {
 	preempt_disable_nested();
 	for (;;) {
-		unsigned n = dir->i_dir_seq;
-		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
+		unsigned n = READ_ONCE(dir->i_dir_seq);
+		if (!(n & 1) && try_cmpxchg(&dir->i_dir_seq, &n, n + 1))
 			return n;
 		cpu_relax();
 	}
-- 
2.50.1