[dm-devel] [PATCH 4/5] blk-mq: introduce blk_get_request_notify

[Ming Lei]

DM-MPATH need to allocate request from underlying queue, but when the
allocation fails, there is no way to make underlying queue's RESTART
to restart DM's queue.

This patch introduces blk_get_request_notify() for this purpose, and
caller need to pass 'wait_queue_entry_t' to this function, and make
sure it is initialized well, so after the current allocation fails,
DM will get notified when there is request available from underlying
queue.

This approach is suggested by Jens, and has been used in blk-mq dispatch
patch for a while, see blk_mq_mark_tag_wait().

Suggested-by: Jens Axboe <axboe at kernel.dk>
Cc: Mike Snitzer <snitzer at redhat.com>
Cc: Laurence Oberman <loberman at redhat.com>
Cc: Bart Van Assche <bart.vanassche at sandisk.com>
Signed-off-by: Ming Lei <ming.lei at redhat.com>
---
 block/blk-mq-tag.c     | 27 ++++++++++++++++++++++++++-
 block/blk-mq.c         | 24 +++++++++++++++++++++---
 block/blk-mq.h         |  1 +
 include/linux/blk-mq.h |  5 +++++
 4 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 336dde07b230..911fc9bd1bab 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -128,10 +128,35 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 	if (tag != -1)
 		goto found_tag;
 
-	if (data->flags & BLK_MQ_REQ_NOWAIT)
+	if ((data->flags & BLK_MQ_REQ_NOWAIT) && !(data->notifier &&
+			(data->flags & BLK_MQ_REQ_ALLOC_NOTIFY)))
 		return BLK_MQ_TAG_FAIL;
 
 	ws = bt_wait_ptr(bt, data->hctx);
+
+	/*
+	 * If caller requires notification when tag is available, add
+	 * wait entry of 'data->notifier' to the wait queue.
+	 */
+	if (data->flags & BLK_MQ_REQ_NOWAIT) {
+		bool added = false;
+
+		spin_lock_irq(&ws->wait.lock);
+		if (list_empty(&data->notifier->entry))
+			__add_wait_queue(&ws->wait, data->notifier);
+		else
+			added = true;
+		spin_unlock_irq(&ws->wait.lock);
+
+		if (added)
+			return BLK_MQ_TAG_FAIL;
+
+		tag = __blk_mq_get_tag(data, bt);
+		if (tag != -1)
+			goto found_tag;
+		return BLK_MQ_TAG_FAIL;
+	}
+
 	drop_ctx = data->ctx == NULL;
 	do {
 		/*
diff --git a/block/blk-mq.c b/block/blk-mq.c
index e91d688792a9..a71e44f17e7f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -384,10 +384,14 @@ static struct request *blk_mq_get_request(struct request_queue *q,
 	return rq;
 }
 
-struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
-		blk_mq_req_flags_t flags)
+static struct request *__blk_mq_alloc_request(struct request_queue *q,
+		unsigned int op, blk_mq_req_flags_t flags,
+		wait_queue_entry_t *notifier)
 {
-	struct blk_mq_alloc_data alloc_data = { .flags = flags };
+	struct blk_mq_alloc_data alloc_data = {
+		.flags = flags,
+		.notifier = notifier,
+	};
 	struct request *rq;
 	int ret;
 
@@ -408,8 +412,22 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
 	rq->bio = rq->biotail = NULL;
 	return rq;
 }
+
+struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
+		blk_mq_req_flags_t flags)
+{
+	return __blk_mq_alloc_request(q, op, flags, NULL);
+}
 EXPORT_SYMBOL(blk_mq_alloc_request);
 
+struct request *blk_mq_alloc_request_notify(struct request_queue *q,
+		unsigned int op, blk_mq_req_flags_t flags,
+		wait_queue_entry_t *notifier)
+{
+	return __blk_mq_alloc_request(q, op, flags, notifier);
+}
+EXPORT_SYMBOL(blk_mq_alloc_request_notify);
+
 struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
 	unsigned int op, blk_mq_req_flags_t flags, unsigned int hctx_idx)
 {
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 88c558f71819..bec2f675f8f1 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -160,6 +160,7 @@ struct blk_mq_alloc_data {
 	struct request_queue *q;
 	blk_mq_req_flags_t flags;
 	unsigned int shallow_depth;
+	wait_queue_entry_t *notifier;
 
 	/* input & output parameter */
 	struct blk_mq_ctx *ctx;
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 8efcf49796a3..335c7dace5a7 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -219,10 +219,15 @@ enum {
 	BLK_MQ_REQ_INTERNAL	= (__force blk_mq_req_flags_t)(1 << 2),
 	/* set RQF_PREEMPT */
 	BLK_MQ_REQ_PREEMPT	= (__force blk_mq_req_flags_t)(1 << 3),
+	/* notify when new rq is available */
+	BLK_MQ_REQ_ALLOC_NOTIFY	= (__force blk_mq_req_flags_t)(1 << 4),
 };
 
 struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
 		blk_mq_req_flags_t flags);
+struct request *blk_mq_alloc_request_notify(struct request_queue *q,
+		unsigned int op, blk_mq_req_flags_t flags,
+		wait_queue_entry_t *notifier);
 struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
 		unsigned int op, blk_mq_req_flags_t flags,
 		unsigned int hctx_idx);
-- 
2.9.5