block: provide helpers for rq_list manipulation

Instead of open-coding the list additions, traversal, and removal,
provide a basic set of helpers.
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -404,17 +404,11 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data,
 		tag = tag_offset + i;
 		tags &= ~(1UL << i);
 		rq = blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
-		rq->rq_next = *data->cached_rq;
-		*data->cached_rq = rq;
+		rq_list_add(data->cached_rq, rq);
 	}
 	data->nr_tags -= nr;
 
-	if (!data->cached_rq)
-		return NULL;
-
-	rq = *data->cached_rq;
-	*data->cached_rq = rq->rq_next;
-	return rq;
+	return rq_list_pop(data->cached_rq);
 }
 
 static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
@@ -622,11 +616,9 @@ EXPORT_SYMBOL_GPL(blk_mq_free_request);
 
 void blk_mq_free_plug_rqs(struct blk_plug *plug)
 {
-	while (plug->cached_rq) {
-		struct request *rq;
+	struct request *rq;
 
-		rq = plug->cached_rq;
-		plug->cached_rq = rq->rq_next;
+	while ((rq = rq_list_pop(&plug->cached_rq)) != NULL) {
 		percpu_ref_get(&rq->q->q_usage_counter);
 		blk_mq_free_request(rq);
 	}
@@ -2418,8 +2410,7 @@ void blk_mq_submit_bio(struct bio *bio)
 
 	plug = blk_mq_plug(q, bio);
 	if (plug && plug->cached_rq) {
-		rq = plug->cached_rq;
-		plug->cached_rq = rq->rq_next;
+		rq = rq_list_pop(&plug->cached_rq);
 		INIT_LIST_HEAD(&rq->queuelist);
 	} else {
 		struct blk_mq_alloc_data data = {

include/linux/blkdev.h

@@ -1298,4 +1298,33 @@ int fsync_bdev(struct block_device *bdev);
 int freeze_bdev(struct block_device *bdev);
 int thaw_bdev(struct block_device *bdev);
 
+#define rq_list_add(listptr, rq)	do {		\
+	(rq)->rq_next = *(listptr);			\
+	*(listptr) = rq;				\
+} while (0)
+
+#define rq_list_pop(listptr)				\
+({							\
+	struct request *__req = NULL;			\
+	if ((listptr) && *(listptr))	{		\
+		__req = *(listptr);			\
+		*(listptr) = __req->rq_next;		\
+	}						\
+	__req;						\
+})
+
+#define rq_list_peek(listptr)				\
+({							\
+	struct request *__req = NULL;			\
+	if ((listptr) && *(listptr))			\
+		__req = *(listptr);			\
+	__req;						\
+})
+
+#define rq_list_for_each(listptr, pos)			\
+	for (pos = rq_list_peek((listptr)); pos; pos = rq_list_next(pos)) \
+
+#define rq_list_next(rq)	(rq)->rq_next
+#define rq_list_empty(list)	((list) == (struct request *) NULL)
+
 #endif /* _LINUX_BLKDEV_H */