block: assert that we're not holding open_mutex over blk_report_disk_dead

blk_report_disk_dead() has the following major callers:

(1) del_gendisk()
(2) blk_mark_disk_dead()

Since del_gendisk() acquires disk->open_mutex it's clear that all
callers are assumed to be called without disk->open_mutex held.
In turn, blk_report_disk_dead() is called without disk->open_mutex held
in del_gendisk().

All callers of blk_mark_disk_dead() call it without disk->open_mutex as
well.

Ensure that it is clear that blk_report_disk_dead() is called without
disk->open_mutex on purpose by asserting it and a comment in the code.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231017184823.1383356-5-hch@lst.deReviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>

block/genhd.c

@@ -559,6 +559,13 @@ static void blk_report_disk_dead(struct gendisk *disk, bool surprise)
 	struct block_device *bdev;
 	unsigned long idx;
 
+	/*
+	 * On surprise disk removal, bdev_mark_dead() may call into file
+	 * systems below. Make it clear that we're expecting to not hold
+	 * disk->open_mutex.
+	 */
+	lockdep_assert_not_held(&disk->open_mutex);
+
 	rcu_read_lock();
 	xa_for_each(&disk->part_tbl, idx, bdev) {
 		if (!kobject_get_unless_zero(&bdev->bd_device.kobj))