blk-mq: Document the functions that iterate over requests

Make it easier to understand the purpose of the functions that iterate
over requests by documenting their purpose. Fix several minor spelling
and grammer mistakes in comments in these functions.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Jianchao Wang <jianchao.w.wang@oracle.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-tag.c

@@ -232,13 +232,26 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 
 	/*
 	 * We can hit rq == NULL here, because the tagging functions
-	 * test and set the bit before assining ->rqs[].
+	 * test and set the bit before assigning ->rqs[].
 	 */
 	if (rq && rq->q == hctx->queue)
 		iter_data->fn(hctx, rq, iter_data->data, reserved);
 	return true;
 }
 
+/**
+ * bt_for_each - iterate over the requests associated with a hardware queue
+ * @hctx:	Hardware queue to examine.
+ * @bt:		sbitmap to examine. This is either the breserved_tags member
+ *		or the bitmap_tags member of struct blk_mq_tags.
+ * @fn:		Pointer to the function that will be called for each request
+ *		associated with @hctx that has been assigned a driver tag.
+ *		@fn will be called as follows: @fn(@hctx, rq, @data, @reserved)
+ *		where rq is a pointer to a request.
+ * @data:	Will be passed as third argument to @fn.
+ * @reserved:	Indicates whether @bt is the breserved_tags member or the
+ *		bitmap_tags member of struct blk_mq_tags.
+ */
 static void bt_for_each(struct blk_mq_hw_ctx *hctx, struct sbitmap_queue *bt,
 			busy_iter_fn *fn, void *data, bool reserved)
 {
@@ -280,6 +293,18 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 	return true;
 }
 
+/**
+ * bt_tags_for_each - iterate over the requests in a tag map
+ * @tags:	Tag map to iterate over.
+ * @bt:		sbitmap to examine. This is either the breserved_tags member
+ *		or the bitmap_tags member of struct blk_mq_tags.
+ * @fn:		Pointer to the function that will be called for each started
+ *		request. @fn will be called as follows: @fn(rq, @data,
+ *		@reserved) where rq is a pointer to a request.
+ * @data:	Will be passed as second argument to @fn.
+ * @reserved:	Indicates whether @bt is the breserved_tags member or the
+ *		bitmap_tags member of struct blk_mq_tags.
+ */
 static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
 			     busy_tag_iter_fn *fn, void *data, bool reserved)
 {
@@ -294,6 +319,15 @@ static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
 		sbitmap_for_each_set(&bt->sb, bt_tags_iter, &iter_data);
 }
 
+/**
+ * blk_mq_all_tag_busy_iter - iterate over all started requests in a tag map
+ * @tags:	Tag map to iterate over.
+ * @fn:		Pointer to the function that will be called for each started
+ *		request. @fn will be called as follows: @fn(rq, @priv,
+ *		reserved) where rq is a pointer to a request. 'reserved'
+ *		indicates whether or not @rq is a reserved request.
+ * @priv:	Will be passed as second argument to @fn.
+ */
 static void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
 		busy_tag_iter_fn *fn, void *priv)
 {
@@ -302,6 +336,15 @@ static void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
 	bt_tags_for_each(tags, &tags->bitmap_tags, fn, priv, false);
 }
 
+/**
+ * blk_mq_tagset_busy_iter - iterate over all started requests in a tag set
+ * @tagset:	Tag set to iterate over.
+ * @fn:		Pointer to the function that will be called for each started
+ *		request. @fn will be called as follows: @fn(rq, @priv,
+ *		reserved) where rq is a pointer to a request. 'reserved'
+ *		indicates whether or not @rq is a reserved request.
+ * @priv:	Will be passed as second argument to @fn.
+ */
 void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 		busy_tag_iter_fn *fn, void *priv)
 {
@@ -314,6 +357,20 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 }
 EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
 
+/**
+ * blk_mq_queue_tag_busy_iter - iterate over all requests with a driver tag
+ * @q:		Request queue to examine.
+ * @fn:		Pointer to the function that will be called for each request
+ *		on @q. @fn will be called as follows: @fn(hctx, rq, @priv,
+ *		reserved) where rq is a pointer to a request and hctx points
+ *		to the hardware queue associated with the request. 'reserved'
+ *		indicates whether or not @rq is a reserved request.
+ * @priv:	Will be passed as third argument to @fn.
+ *
+ * Note: if @q->tag_set is shared with other request queues then @fn will be
+ * called for all requests on all queues that share that tag set and not only
+ * for requests associated with @q.
+ */
 void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 		void *priv)
 {
@@ -321,11 +378,11 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 	int i;
 
 	/*
-	 * __blk_mq_update_nr_hw_queues will update the nr_hw_queues and
-	 * queue_hw_ctx after freeze the queue. So we could use q_usage_counter
-	 * to avoid race with it. __blk_mq_update_nr_hw_queues will users
-	 * synchronize_rcu to ensure all of the users go out of the critical
-	 * section below and see zeroed q_usage_counter.
+	 * __blk_mq_update_nr_hw_queues() updates nr_hw_queues and queue_hw_ctx
+	 * while the queue is frozen. So we can use q_usage_counter to avoid
+	 * racing with it. __blk_mq_update_nr_hw_queues() uses
+	 * synchronize_rcu() to ensure this function left the critical section
+	 * below.
 	 */
 	rcu_read_lock();
 	if (percpu_ref_is_zero(&q->q_usage_counter)) {
@@ -337,7 +394,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 		struct blk_mq_tags *tags = hctx->tags;
 
 		/*
-		 * If not software queues are currently mapped to this
+		 * If no software queues are currently mapped to this
 		 * hardware queue, there's nothing to check
 		 */
 		if (!blk_mq_hw_queue_mapped(hctx))