[PATCH V2 1/3] blk-mq: add API of blk_mq_queue_frozen

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

blk_mq_freeze_queue_wait() isn't very flexible for some case, such as
error recovery: when blk_mq_freeze_queue_wait is called in error
recovery handler, new problem may be triggered on this controller, so
in-flight IO may not complete when blk_mq_freeze_queue_wait() is called.

And error recovery is often run in single context, so dead lock may be
triggered, because error recover handler can't move on.

Add one new API of blk_mq_queue_frozen(), error recovery handler may
use this helper to query if the queue has been frozen completely.
Meantime, the error recovery handler can check if there is hardware
failure happened. If yes, error recovery handler can break from current
handling, and run a fresh new recovery, so deadlock can be avoided.

This API will be used to improve error handling of nvme-pci's timeout
handler.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c         | 6 ++++++
 include/linux/blk-mq.h | 1 +
 2 files changed, 7 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a5ff8140e52c..dc4e73d7ad02 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -148,6 +148,12 @@ void blk_mq_freeze_queue_wait(struct request_queue *q)
 }
 EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_wait);
 
+bool blk_mq_queue_frozen(struct request_queue *q)
+{
+	return percpu_ref_is_zero(&q->q_usage_counter);
+}
+EXPORT_SYMBOL_GPL(blk_mq_queue_frozen);
+
 int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,
 				     unsigned long timeout)
 {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 43e38d21ca4a..1f867ad02c5c 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -518,6 +518,7 @@ void blk_freeze_queue_start(struct request_queue *q);
 void blk_mq_freeze_queue_wait(struct request_queue *q);
 int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,
 				     unsigned long timeout);
+bool blk_mq_queue_frozen(struct request_queue *q);
 
 int blk_mq_map_queues(struct blk_mq_queue_map *qmap);
 void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues);
-- 
2.25.2


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