raid5-cache: use try_cmpxchg in r5l_wake_reclaim

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@kernel.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>

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);
 }