linux-kernel - [PATCH] fs/xfs: Use atomic64_try_cmpxchg in xlog_grant_{add,sub}

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Message-Id: <20220809165615.9694-1-ubizjak@gmail.com>
Date:   Tue,  9 Aug 2022 18:56:15 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        "Darrick J. Wong" <djwong@...nel.org>
Subject: [PATCH] fs/xfs: Use atomic64_try_cmpxchg in xlog_grant_{add,sub}_space

Use `!atomic64_try_cmpxchg(ptr, &old, new)` instead of
`atomic64_cmpxchg(ptr, old, new) != old` in xlog_grant_{add,sub}_space.
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.

- atomic64_try_cmpxchg implicitly assigns the *ptr value to &old when
  cmpxchg fails, enabling further code simplifications.

This patch has no functional change.

Cc: "Darrick J. Wong" <djwong@...nel.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 fs/xfs/xfs_log.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 4b1c0a9c6368..92e39873d09e 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -148,7 +148,7 @@ xlog_grant_sub_space(
 	int			bytes)
 {
 	int64_t	head_val = atomic64_read(head);
-	int64_t new, old;
+	int64_t new;
 
 	do {
 		int	cycle, space;
@@ -161,10 +161,9 @@ xlog_grant_sub_space(
 			cycle--;
 		}
 
-		old = head_val;
 		new = xlog_assign_grant_head_val(cycle, space);
-		head_val = atomic64_cmpxchg(head, old, new);
-	} while (head_val != old);
+
+	} while (!atomic64_try_cmpxchg(head, &head_val, new));
 }
 
 static void
@@ -174,7 +173,7 @@ xlog_grant_add_space(
 	int			bytes)
 {
 	int64_t	head_val = atomic64_read(head);
-	int64_t new, old;
+	int64_t new;
 
 	do {
 		int		tmp;
@@ -190,10 +189,9 @@ xlog_grant_add_space(
 			cycle++;
 		}
 
-		old = head_val;
 		new = xlog_assign_grant_head_val(cycle, space);
-		head_val = atomic64_cmpxchg(head, old, new);
-	} while (head_val != old);
+
+	} while (!atomic64_try_cmpxchg(head, &head_val, new));
 }
 
 STATIC void
-- 
2.37.1