linux-kernel - [PATCH RESEND v2] sbitmap: Use atomic_long_try_cmpxchg in __sbitmap_queue_get

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Message-Id: <20220908151200.9993-1-ubizjak@gmail.com>
Date:   Thu,  8 Sep 2022 17:12:00 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Jens Axboe <axboe@...nel.dk>
Subject: [PATCH RESEND v2] 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 <axboe@...nel.dk>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
v2: Resend against linux-block for-6.1/block.
---
 lib/sbitmap.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index a39b1a877366..299765a027bf 100644
--- a/lib/sbitmap.c
+++ b/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,
-- 
2.37.2