sbitmap: Use atomic_long_try_cmpxchg in __sbitmap_queue_get_batch

Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in __sbitmap_queue_get_batch.
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_long_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications, e.g.
an extra memory read can be avoided in the loop.

No functional change intended.

Cc: Jens Axboe <[email protected]>
Signed-off-by: Uros Bizjak <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

lib/sbitmap.c

@@ -533,16 +533,16 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
 		nr = find_first_zero_bit(&map->word, map_depth);
 		if (nr + nr_tags <= map_depth) {
 			atomic_long_t *ptr = (atomic_long_t *) &map->word;
-			unsigned long val, ret;
+			unsigned long val;
 
 			get_mask = ((1UL << nr_tags) - 1) << nr;
+			val = READ_ONCE(map->word);
 			do {
-				val = READ_ONCE(map->word);
 				if ((val & ~get_mask) != val)
 					goto next;
-				ret = atomic_long_cmpxchg(ptr, val, get_mask | val);
-			} while (ret != val);
-			get_mask = (get_mask & ~ret) >> nr;
+			} while (!atomic_long_try_cmpxchg(ptr, &val,
+							  get_mask | val));
+			get_mask = (get_mask & ~val) >> nr;
 			if (get_mask) {
 				*offset = nr + (index << sb->shift);
 				update_alloc_hint_after_get(sb, depth, hint,