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
