[PATCH v2] raid5-cache: use try_cmpxchg in r5l_wake_reclaim

From: Uros Bizjak
Date: Thu Oct 20 2022 - 11:51:30 EST


Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
r5l_wake_reclaim. 86 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@xxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
v2: Fix garbled subject line
---
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