bdev_mark_dead()'s @surprise == true means the device is already gone.
The filesystem callback fs_bdev_mark_dead() honours this and skips
sync_filesystem(), but the bare block device path (no ->mark_dead op)
lost its !surprise guard when the holder ->mark_dead callback was wired
up (see Fixes), and now calls sync_blockdev() unconditionally, which can
hang forever waiting on writeback that can no longer complete.

syzkaller hit this via nvme_reset_work()'s "I/O queues lost" path:
nvme_mark_namespaces_dead() -> blk_mark_disk_dead() ->
bdev_mark_dead(bdev, true) -> sync_blockdev() blocks in
folio_wait_writeback(), wedging the reset worker and every task waiting
on it.

Skip the sync on surprise removal, matching fs_bdev_mark_dead();
invalidate_bdev() still runs. Orderly removal (surprise == false) is
unchanged.

Fixes: d8530de5a6e8 ("block: call into the file system for bdev_mark_dead")
Found by FuzzNvme(Syzkaller with FEMU fuzzing framework).
Acked-by: Sungwoo Kim <iam@sung-woo.kim>
Acked-by: Dave Tian <daveti@purdue.edu>
Acked-by: Weidong Zhu <weizhu@fiu.edu>
Signed-off-by: Chao Shi <coshi036@gmail.com>
---
 block/bdev.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/bdev.c b/block/bdev.c
index b8fbb9576110..7fc3f5ba22a3 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -1259,7 +1259,13 @@ void bdev_mark_dead(struct block_device *bdev, bool surprise)
 		bdev->bd_holder_ops->mark_dead(bdev, surprise);
 	else {
 		mutex_unlock(&bdev->bd_holder_lock);
-		sync_blockdev(bdev);
+		/*
+		 * On surprise removal the device is already gone; syncing is
+		 * futile and can hang forever waiting on I/O that will never
+		 * complete.  Match fs_bdev_mark_dead(), which also skips it.
+		 */
+		if (!surprise)
+			sync_blockdev(bdev);
 	}
 
 	invalidate_bdev(bdev);
-- 
2.43.0