block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old in atomic_inc_below. 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, atomic_try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg fails, enabling further code simplifications. No functional change intended. Signed-off-by: Uros Bizjak <[email protected]> Cc: Jens Axboe <[email protected]> Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Jens Axboe <[email protected]>
josf · Jul 12, 2022 · f4b1e27 · f4b1e27
1 parent f3ec5d1
commit f4b1e27
Showing 1 changed file with 2 additions and 8 deletions.
diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
@@ -10,16 +10,10 @@ static bool atomic_inc_below(atomic_t *v, unsigned int below)
 {
 	unsigned int cur = atomic_read(v);
 
-	for (;;) {
-		unsigned int old;
-
+	do {
 		if (cur >= below)
 			return false;
-		old = atomic_cmpxchg(v, cur, cur + 1);
-		if (old == cur)
-			break;
-		cur = old;
-	}
+	} while (!atomic_try_cmpxchg(v, &cur, cur + 1));
 
 	return true;
 }