linux-kernel - [PATCH] raid5-cache: use try_cmpxchg in r5l_wake_reclaim Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in r5l_wake_reclaim. x86 CMPXCHG instruction returns success in ZF flag, so this change saves a compare after cmpxchg (and related move instruction in front of cmpxchg).

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Message-Id: <20221020153034.7905-1-ubizjak@gmail.com>
Date:   Thu, 20 Oct 2022 17:30:34 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Song Liu <song@...nel.org>
Subject: [PATCH] raid5-cache: use try_cmpxchg in r5l_wake_reclaim Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in r5l_wake_reclaim.  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.

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.

Cc: Song Liu <song@...nel.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 drivers/md/raid5-cache.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 832d8566e165..a63023aae21e 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -1565,11 +1565,12 @@ void r5l_wake_reclaim(struct r5l_log *log, sector_t space)

 	if (!log)
 		return;
+
+	target = READ_ONCE(log->reclaim_target);
 	do {
-		target = log->reclaim_target;
 		if (new < target)
 			return;
-	} while (cmpxchg(&log->reclaim_target, target, new) != target);
+	} while (!try_cmpxchg(&log->reclaim_target, &target, new));
 	md_wakeup_thread(log->reclaim_thread);
 }

-- 
2.37.3