linux-kernel - [PATCH 2/4] sbitmap: remove redundant check in __sbitmap_queue_get

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Message-Id: <20221208220147.3315388-3-shikemeng@huaweicloud.com>
Date:   Fri,  9 Dec 2022 06:01:45 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linfeilong@...wei.com, liuzhiqiang26@...wei.com
Subject: [PATCH 2/4] sbitmap: remove redundant check in __sbitmap_queue_get_batch

Commit fbb564a557809 ("lib/sbitmap: Fix invalid loop in
__sbitmap_queue_get_batch()") mentioned that "Checking free bits when
setting the target bits. Otherwise, it may reuse the busying bits."
This commit add check to make sure all masked bits in word before
cmpxchg is zero. Then the existing check after cmpxchg to check any
zero bit is existing in masked bits in word is redundant.

Actually, old value of word before cmpxchg is stored in val and we
will filter out busy bits in val by "(get_mask & ~val)" after cmpxchg.
So we will not reuse busy bits methioned in commit fbb564a557809
("lib/sbitmap: Fix invalid loop in __sbitmap_queue_get_batch()"). Revert
new-added check to remove redundant check.

Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
---
 lib/sbitmap.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index cb5e03a2d65b..11e75f4040fb 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -518,11 +518,9 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,

 			get_mask = ((1UL << nr_tags) - 1) << nr;
 			val = READ_ONCE(map->word);
-			do {
-				if ((val & ~get_mask) != val)
-					goto next;
-			} while (!atomic_long_try_cmpxchg(ptr, &val,
-							  get_mask | val));
+			while (!atomic_long_try_cmpxchg(ptr, &val,
+							  get_mask | val))
+				;
 			get_mask = (get_mask & ~val) >> nr;
 			if (get_mask) {
 				*offset = nr + (index << sb->shift);
-- 
2.30.0