blk-mq: bitmap tag: select random tag betweet 0 and (depth - 1)

ming1 · axboe · commit 9d3d21aeb4f1 · 2014-05-10T15:43:14.000-06:00
The selected tag should be selected at random between 0 and
(depth - 1) with probability 1/depth, instead between 0 and
(depth - 2) with probability 1/(depth - 1).

Signed-off-by: Ming Lei &lt;tom.leiming@gmail.com&gt;
Signed-off-by: Jens Axboe &lt;axboe@fb.com&gt;
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
@@ -449,10 +449,7 @@ void blk_mq_tag_init_last_tag(struct blk_mq_tags *tags, unsigned int *tag)
 {
 	unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
 
-	if (depth > 1)
-		*tag = prandom_u32() % (depth - 1);
-	else
-		*tag = 0;
+	*tag = prandom_u32() % depth;
 }
 
 ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)

Original file line number	Diff line number	Diff line change
`@@ -449,10 +449,7 @@ void blk_mq_tag_init_last_tag(struct blk_mq_tags tags, unsigned int tag)`
`449`	`449`	`{`
`450`	`450`	`unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;`
`451`	`451`
`452`		`- if (depth > 1)`
`453`		`- *tag = prandom_u32() % (depth - 1);`
`454`		`- else`
`455`		`- *tag = 0;`
	`452`	`+ *tag = prandom_u32() % depth;`
`456`	`453`	`}`
`457`	`454`
`458`	`455`	`ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags tags, char page)`