linux-ext4 - [PATCH] fs/ext4: Use try_cmpxchg in ext4_update_bh

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Date:   Tue,  9 Aug 2022 18:46:26 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Subject: [PATCH] fs/ext4: Use try_cmpxchg in ext4_update_bh_state

Use `!try_cmpxchg(ptr, &old, new)` instead of
`cmpxchg(ptr, old, new) != old` in ext4_update_bh_state. This
has two benefits:

- The x86 cmpxchg instruction returns success in the ZF flag, so this
  change saves a compare after cmpxchg, as well as a related move
  instruction in the front of cmpxchg.

- atomic_try_cmpxchg implicitly assigns the *ptr value to &old when
  cmpxchg fails, enabling further code simplifications.

This patch has no functional change.

Cc: "Theodore Ts'o" <tytso@....edu>
Cc: Andreas Dilger <adilger.kernel@...ger.ca>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 fs/ext4/inode.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 601214453c3a..ce2f03621be8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -780,11 +780,10 @@ 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,
-- 
2.37.1