[PATCH 4/4] blk-mq: use blk_queue_enter/exit to protect debugfs file creation

[Yu Kuai]

Replace the freeze depth check with blk_queue_enter()/blk_queue_exit()
which properly waits for the queue to be unfrozen and prevents new
freezes while creating debugfs files. This provides correct
synchronization without false warnings.

If the queue is dying (blk_queue_enter returns error), skip creating
the debugfs files as the queue is being torn down anyway.

Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki at wdc.com>
Closes: https://lore.kernel.org/all/aYWQR7CtYdk3K39g@shinmob/
Signed-off-by: Yu Kuai <yukuai at fnnas.com>
---
 block/blk-mq-debugfs.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index faeaa1fc86a7..03583d0d3972 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -613,11 +613,6 @@ static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
 				 const struct blk_mq_debugfs_attr *attr)
 {
 	lockdep_assert_held(&q->debugfs_mutex);
-	/*
-	 * Creating new debugfs entries with queue freezed has the risk of
-	 * deadlock.
-	 */
-	WARN_ON_ONCE(q->mq_freeze_depth != 0);
 	/*
 	 * debugfs_mutex should not be nested under other locks that can be
 	 * grabbed while queue is frozen.
@@ -628,9 +623,19 @@ static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
 	if (IS_ERR_OR_NULL(parent))
 		return;
 
+	/*
+	 * Avoid creating debugfs files while the queue is frozen, wait for
+	 * the queue to be unfrozen and prevent new freeze while creating
+	 * debugfs files.
+	 */
+	if (blk_queue_enter(q, 0))
+		return;
+
 	for (; attr->name; attr++)
 		debugfs_create_file_aux(attr->name, attr->mode, parent,
 				    (void *)attr, data, &blk_mq_debugfs_fops);
+
+	blk_queue_exit(q);
 }
 
 void blk_mq_debugfs_register(struct request_queue *q)
-- 
2.51.0