fs/ext4: use try_cmpxchg in ext4_update_bh_state

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
ext4_update_bh_state.  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.  There is no need to re-read the value in the loop.

No functional change intended.

Link: https://lkml.kernel.org/r/20221102071147.6642-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 9d9f414f99fec31941ca4a3efb27565156e8914e..dc8da7bbc240a16d02d6d661f07db94ee9a1d706 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -786,11 +786,10 @@ static void ext4_update_bh_state(struct buffer_head *bh, unsigned long flags)
         * once we get rid of using bh as a container for mapping information
         * to pass to / from get_block functions, this can go away.
         */
+       old_state = READ_ONCE(bh->b_state);
        do {
-               old_state = READ_ONCE(bh->b_state);
                new_state = (old_state & ~EXT4_MAP_FLAGS) | flags;
-       } while (unlikely(
-                cmpxchg(&bh->b_state, old_state, new_state) != old_state));
+       } while (unlikely(!try_cmpxchg(&bh->b_state, &old_state, new_state)));
 }
 
 static int _ext4_get_block(struct inode *inode, sector_t iblock,