[PATCH 5/6] blk-mq: disable preempt during allocating request tag

[Ming Lei <ming.lei@redhat.com>]

Disable preempt for a little while during allocating request tag, so
request's tag(internal tag) is always allocated on the cpu of data->ctx,
prepare for improving to handle cpu hotplug during allocating request.

In the following patch, hctx->state will be checked to see if it becomes
inactive which is always set on the last CPU of hctx->cpumask.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Garry <john.garry@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-tag.c | 4 ++++
 block/blk-mq.c     | 4 ++++
 block/blk-mq.h     | 1 +
 3 files changed, 9 insertions(+)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index c27c6dfc7d36..0ae79ca6e2da 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -152,11 +152,15 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		if (tag != -1)
 			break;
 
+		if (data->preempt_disabled)
+			preempt_enable();
 		bt_prev = bt;
 		io_schedule();
 
 		sbitmap_finish_wait(bt, ws, &wait);
 
+		if (data->preempt_disabled)
+			preempt_disable();
 		data->ctx = blk_mq_get_ctx(data->q);
 		data->hctx = blk_mq_map_queue(data->q, data->cmd_flags,
 						data->ctx);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 151b63b1f88b..18999198a37f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -365,6 +365,9 @@ static struct request *blk_mq_get_request(struct request_queue *q,
 	data->q = q;
 
 	WARN_ON_ONCE(data->ctx || data->hctx);
+
+	preempt_disable();
+	data->preempt_disabled = true;
 	data->ctx = blk_mq_get_ctx(q);
 	data->hctx = blk_mq_map_queue(q, data->cmd_flags, data->ctx);
 	if (data->cmd_flags & REQ_NOWAIT)
@@ -387,6 +390,7 @@ static struct request *blk_mq_get_request(struct request_queue *q,
 	}
 
 	tag = blk_mq_get_tag(data);
+	preempt_enable();
 	if (tag == BLK_MQ_TAG_FAIL) {
 		data->ctx = NULL;
 		data->hctx = NULL;
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 10bfdfb494fa..5ebc8d8d5a54 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -152,6 +152,7 @@ struct blk_mq_alloc_data {
 	blk_mq_req_flags_t flags;
 	unsigned int shallow_depth;
 	unsigned int cmd_flags;
+	bool preempt_disabled;
 
 	/* input & output parameter */
 	struct blk_mq_ctx *ctx;
-- 
2.25.2