[PATCH 2/2] block: mark the disk dead before taking open_mutx in del_gendisk

[Christoph Hellwig <hch@lst.de>]

Now that we stop sd and sr from submitting passthrough commands from
their ->release methods we can and should start the drain before taking
->open_mutex, so that we can entirely prevent this kind of deadlock by
ensuring that the disk is clearly marked dead before open_mutex is
taken in del_gendisk.

This includes a revert of commit 7e04da2dc701 ("block: fix deadlock
between sd_remove & sd_release"), which was a partial fix for a similar
deadlock.

Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 block/genhd.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 7026569fa8a0be..c15e8f1163664b 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -655,16 +655,6 @@ void del_gendisk(struct gendisk *disk)
 	if (WARN_ON_ONCE(!disk_live(disk) && !(disk->flags & GENHD_FL_HIDDEN)))
 		return;
 
-	disk_del_events(disk);
-
-	/*
-	 * Prevent new openers by unlinked the bdev inode.
-	 */
-	mutex_lock(&disk->open_mutex);
-	xa_for_each(&disk->part_tbl, idx, part)
-		bdev_unhash(part);
-	mutex_unlock(&disk->open_mutex);
-
 	/*
 	 * Tell the file system to write back all dirty data and shut down if
 	 * it hasn't been notified earlier.
@@ -673,10 +663,22 @@ void del_gendisk(struct gendisk *disk)
 		blk_report_disk_dead(disk, false);
 
 	/*
-	 * Drop all partitions now that the disk is marked dead.
+	 * Then mark the disk dead to stop new requests from being served ASAP.
+	 * This needs to happen before taking ->open_mutex to prevent deadlocks
+	 * with SCSI ULPs that send passthrough commands from their ->release
+	 * methods.
 	 */
-	mutex_lock(&disk->open_mutex);
 	__blk_mark_disk_dead(disk);
+
+	disk_del_events(disk);
+
+	/*
+	 * Prevent new openers by unlinking the bdev inode, and drop all
+	 * partitions.
+	 */
+	mutex_lock(&disk->open_mutex);
+	xa_for_each(&disk->part_tbl, idx, part)
+		bdev_unhash(part);
 	xa_for_each_start(&disk->part_tbl, idx, part, 1)
 		drop_partition(part);
 	mutex_unlock(&disk->open_mutex);
-- 
2.45.2