linux-kernel - [PATCH] fs/btrfs: Use atomic_try_cmpxchg in free_extent

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Message-Id: <20220809163633.8255-1-ubizjak@gmail.com>
Date:   Tue,  9 Aug 2022 18:36:33 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>
Subject: [PATCH] fs/btrfs: Use atomic_try_cmpxchg in free_extent_buffer

Use `atomic_try_cmpxchg(ptr, &old, new)` instead of
`atomic_cmpxchg(ptr, old, new) == old` in free_extent_buffer. 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: Chris Mason <clm@...com>
Cc: Josef Bacik <josef@...icpanda.com>
Cc: David Sterba <dsterba@...e.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 fs/btrfs/extent_io.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index bfae67c593c5..15ff196cbd6d 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -6328,18 +6328,16 @@ static int release_extent_buffer(struct extent_buffer *eb)
 void free_extent_buffer(struct extent_buffer *eb)
 {
 	int refs;
-	int old;
 	if (!eb)
 		return;
 
+	refs = atomic_read(&eb->refs);
 	while (1) {
-		refs = atomic_read(&eb->refs);
 		if ((!test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && refs <= 3)
 		    || (test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) &&
 			refs == 1))
 			break;
-		old = atomic_cmpxchg(&eb->refs, refs, refs - 1);
-		if (old == refs)
+		if (atomic_try_cmpxchg(&eb->refs, &refs, refs - 1))
 			return;
 	}
 
-- 
2.37.1