linux-kernel - [PATCH 1/6] sbitmap: fix hint wrap in the failure case

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Date:   Thu, 20 Jul 2023 17:45:50 +0800
From:   chengming.zhou@...ux.dev
To:     axboe@...nel.dk, osandov@...com, ming.lei@...hat.com,
        kbusch@...nel.org, krisman@...e.de
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        zhouchengming@...edance.com
Subject: [PATCH 1/6] sbitmap: fix hint wrap in the failure case

From: Chengming Zhou <zhouchengming@...edance.com>

```
hint = nr + 1;
if (hint >= depth - 1)
	hint = 0;
```

Now we wrap the hint to 0 in the failure case, but:
1. hint == depth - 1, is actually an available offset hint, which
   we shouldn't wrap hint to 0.
2. In the strict round_robin non-wrap case, we shouldn't wrap at all.

```
wrap = wrap && hint;
```

We only need to check wrap based on the original hint ( > 0), don't need
to recheck the new hint which maybe updated in the failure case.
Also delete the mismatched comments by the way.

Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 lib/sbitmap.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index eff4e42c425a..5ed6c2adf58e 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -144,12 +144,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
 	while (1) {
 		nr = find_next_zero_bit(word, depth, hint);
 		if (unlikely(nr >= depth)) {
-			/*
-			 * We started with an offset, and we didn't reset the
-			 * offset to 0 in a failure case, so start from 0 to
-			 * exhaust the map.
-			 */
-			if (hint && wrap) {
+			if (wrap) {
 				hint = 0;
 				continue;
 			}
@@ -160,8 +155,13 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
 			break;
 
 		hint = nr + 1;
-		if (hint >= depth - 1)
-			hint = 0;
+		if (hint >= depth) {
+			if (wrap) {
+				hint = 0;
+				continue;
+			}
+			return -1;
+		}
 	}
 
 	return nr;
-- 
2.41.0