[RFC PATCH 1/3] blk-mq: Reference count request usage

[Keith Busch <keith.busch@xxxxxxxxx>]

This patch adds a struct kref to struct request so that request users
can be sure they're operating on the same request without it changing
while they're processing it. The request's tag won't be released for
reuse until the last user is done with it.

Signed-off-by: Keith Busch <keith.busch@xxxxxxxxx>
---
 block/blk-mq.c         | 30 +++++++++++++++++++++++-------
 include/linux/blkdev.h |  2 ++
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4cbfd784e837..8b370ed75605 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -332,6 +332,7 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 #endif
 
 	data->ctx->rq_dispatched[op_is_sync(op)]++;
+	kref_init(&rq->ref);
 	return rq;
 }
 
@@ -465,13 +466,33 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
 }
 EXPORT_SYMBOL_GPL(blk_mq_alloc_request_hctx);
 
+static void blk_mq_exit_request(struct kref *ref)
+{
+	struct request *rq = container_of(ref, struct request, ref);
+	struct request_queue *q = rq->q;
+	struct blk_mq_ctx *ctx = rq->mq_ctx;
+	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
+	const int sched_tag = rq->internal_tag;
+
+	if (rq->tag != -1)
+		blk_mq_put_tag(hctx, hctx->tags, ctx, rq->tag);
+	if (sched_tag != -1)
+		blk_mq_put_tag(hctx, hctx->sched_tags, ctx, sched_tag);
+	blk_mq_sched_restart(hctx);
+	blk_queue_exit(q);
+}
+
+static void blk_mq_put_request(struct request *rq)
+{
+       kref_put(&rq->ref, blk_mq_exit_request);
+}
+
 void blk_mq_free_request(struct request *rq)
 {
 	struct request_queue *q = rq->q;
 	struct elevator_queue *e = q->elevator;
 	struct blk_mq_ctx *ctx = rq->mq_ctx;
 	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
-	const int sched_tag = rq->internal_tag;
 
 	if (rq->rq_flags & RQF_ELVPRIV) {
 		if (e && e->type->ops.mq.finish_request)
@@ -495,12 +516,7 @@ void blk_mq_free_request(struct request *rq)
 		blk_put_rl(blk_rq_rl(rq));
 
 	blk_mq_rq_update_state(rq, MQ_RQ_IDLE);
-	if (rq->tag != -1)
-		blk_mq_put_tag(hctx, hctx->tags, ctx, rq->tag);
-	if (sched_tag != -1)
-		blk_mq_put_tag(hctx, hctx->sched_tags, ctx, sched_tag);
-	blk_mq_sched_restart(hctx);
-	blk_queue_exit(q);
+	blk_mq_put_request(rq);
 }
 EXPORT_SYMBOL_GPL(blk_mq_free_request);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index f3999719f828..26bf2c1e3502 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -257,6 +257,8 @@ struct request {
 	struct u64_stats_sync aborted_gstate_sync;
 	u64 aborted_gstate;
 
+	struct kref ref;
+
 	/* access through blk_rq_set_deadline, blk_rq_deadline */
 	unsigned long __deadline;
 
-- 
2.14.3