A recent lockdep[1] splat observed while running blktest block/005
reveals a potential deadlock caused by the cpu_hotplug_lock dependency
on ->freeze_lock. This dependency was introduced by commit 033b667a823e
("block: blk-rq-qos: guard rq-qos helpers by static key").

That change added a static key to avoid fetching q->rq_qos when
neither blk-wbt nor blk-iolatency is configured. The static key
dynamically patches kernel text to a NOP when disabled, eliminating
overhead of fetching q->rq_qos in the I/O hot path. However, enabling
a static key at runtime requires acquiring both cpu_hotplug_lock and
jump_label_mutex. When this happens after the queue has already been
frozen (i.e., while holding ->freeze_lock), it creates a locking
dependency from cpu_hotplug_lock to ->freeze_lock, which leads to a
potential deadlock reported by lockdep [1].

To resolve this, replace the static key mechanism with q->queue_flags:
QUEUE_FLAG_QOS_ENABLED. This flag is evaluated in the fast path before
accessing q->rq_qos. If the flag is set, we proceed to fetch q->rq_qos;
otherwise, the access is skipped.

Since q->queue_flags is commonly accessed in IO hotpath and resides in
the first cacheline of struct request_queue, checking it imposes minimal
overhead while eliminating the deadlock risk.

This change avoids the lockdep splat without introducing performance
regressions.

[1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/ (local)

Reported-by: Shinichiro Kawasaki <redacted>
Closes: https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/ (local)
Fixes: 033b667a823e ("block: blk-rq-qos: guard rq-qos helpers by static key")
Tested-by: Shin'ichiro Kawasaki <redacted>
Signed-off-by: Nilay Shroff <redacted>
---
 block/blk-mq-debugfs.c |  1 +
 block/blk-rq-qos.c     |  9 ++++---
 block/blk-rq-qos.h     | 54 ++++++++++++++++++++++++------------------
 include/linux/blkdev.h |  1 +
 4 files changed, 37 insertions(+), 28 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 7ed3e71f2fc0..32c65efdda46 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c

@@ -95,6 +95,7 @@ static const char *const blk_queue_flag_name[] = {
 	QUEUE_FLAG_NAME(SQ_SCHED),
 	QUEUE_FLAG_NAME(DISABLE_WBT_DEF),
 	QUEUE_FLAG_NAME(NO_ELV_SWITCH),
+	QUEUE_FLAG_NAME(QOS_ENABLED),
 };
 #undef QUEUE_FLAG_NAME
 

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index b1e24bb85ad2..654478dfbc20 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c

@@ -2,8 +2,6 @@
 
 #include "blk-rq-qos.h"
 
-__read_mostly DEFINE_STATIC_KEY_FALSE(block_rq_qos);
-
 /*
  * Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
  * false if 'v' + 1 would be bigger than 'below'.

@@ -319,8 +317,8 @@ void rq_qos_exit(struct request_queue *q)
 		struct rq_qos *rqos = q->rq_qos;
 		q->rq_qos = rqos->next;
 		rqos->ops->exit(rqos);
-		static_branch_dec(&block_rq_qos);
 	}
+	blk_queue_flag_clear(QUEUE_FLAG_QOS_ENABLED, q);
 	mutex_unlock(&q->rq_qos_mutex);
 }
 

@@ -346,7 +344,7 @@ int rq_qos_add(struct rq_qos *rqos, struct gendisk *disk, enum rq_qos_id id,
 		goto ebusy;
 	rqos->next = q->rq_qos;
 	q->rq_qos = rqos;
-	static_branch_inc(&block_rq_qos);
+	blk_queue_flag_set(QUEUE_FLAG_QOS_ENABLED, q);