blk-mq: use NOIO context to prevent deadlock during debugfs creation

Creating debugfs entries can trigger fs reclaim, which can enter back
into the block layer request_queue. This can cause deadlock if the
queue is frozen.

Previously, a WARN_ON_ONCE check was used in debugfs_create_files()
to detect this condition, but it was racy since the queue can be frozen
from another context at any time.

Introduce blk_debugfs_lock()/blk_debugfs_unlock() helpers that combine
the debugfs_mutex with memalloc_noio_save()/restore() to prevent fs
reclaim from triggering block I/O. Also add blk_debugfs_lock_nomemsave()
and blk_debugfs_unlock_nomemrestore() variants for callers that don't
need NOIO protection (e.g., debugfs removal or read-only operations).

Replace all raw debugfs_mutex lock/unlock pairs with these helpers,
using the _nomemsave/_nomemrestore variants where appropriate.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Closes: https://lore.kernel.org/all/CAHj4cs9gNKEYAPagD9JADfO5UH+OiCr4P7OO2wjpfOYeM-RV=A@mail.gmail.com/
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Closes: https://lore.kernel.org/all/aYWQR7CtYdk3K39g@shinmob/
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 31 insertions, 0 deletions

diff --git a/block/blk.h b/block/blk.h
index a6b1de509733..f6053e9dd2aa 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -729,4 +729,35 @@ static inline void blk_unfreeze_release_lock(struct request_queue *q)
 }
 #endif
 
+/*
+ * debugfs directory and file creation can trigger fs reclaim, which can enter
+ * back into the block layer request_queue. This can cause deadlock if the
+ * queue is frozen. Use NOIO context together with debugfs_mutex to prevent fs
+ * reclaim from triggering block I/O.
+ */
+static inline void blk_debugfs_lock_nomemsave(struct request_queue *q)
+{
+	mutex_lock(&q->debugfs_mutex);
+}
+
+static inline void blk_debugfs_unlock_nomemrestore(struct request_queue *q)
+{
+	mutex_unlock(&q->debugfs_mutex);
+}
+
+static inline unsigned int __must_check blk_debugfs_lock(struct request_queue *q)
+{
+	unsigned int memflags = memalloc_noio_save();
+
+	blk_debugfs_lock_nomemsave(q);
+	return memflags;
+}
+
+static inline void blk_debugfs_unlock(struct request_queue *q,
+				      unsigned int memflags)
+{
+	blk_debugfs_unlock_nomemrestore(q);
+	memalloc_noio_restore(memflags);
+}
+
 #endif /* BLK_INTERNAL_H */