Re: INFO: task hung in __blkdev_get

[Tetsuo Handa]

Hello.

Since syzbot is reporting so many hung up bug which involves /dev/loopX ,
is it possible to "temporarily" apply below patch for testing under syzbot
( after "block/loop: fix deadlock after loop_set_status" and
"loop: fix LOOP_GET_STATUS lock imbalance" in linux-block.git#for-linus
are merged into linux.git )?

I haven't got a smoking gun by lockdep, but I noticed that

  [upstream] INFO: task hung in lo_open (2)
  https://syzkaller.appspot.com/bug?id=1f93b57f496d969efb9fb24167f6f9de5ee068fd

contained "lo->lo_ctl_mutex => bdev->bd_mutex" locking order

  2 locks held by syz-executor6/15084:
   #0:  (&lo->lo_ctl_mutex/1){+.+.}, at: [<00000000cc154b8d>] lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355
   #1:  (&bdev->bd_mutex){+.+.}, at: [<0000000058b7c5b5>] blkdev_reread_part+0x1e/0x40 block/ioctl.c:192

while commit f028f3b2f987ebc6 ("loop: fix circular locking in loop_clr_fd()")
says that

  * Calling fput holding lo_ctl_mutex triggers a circular
  * lock dependency possibility warning as fput can take
  * bd_mutex which is usually taken before lo_ctl_mutex.

which implies that the locking order should be "bdev->bd_mutex => lo->lo_ctl_mutex"
and also says that use of "_nested" version might mask some other real bugs
which could be the bug syzbot is frequently reporting as hung tasks...

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 264abaa..5559b15 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1360,7 +1360,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
 	struct loop_device *lo = bdev->bd_disk->private_data;
 	int err;
 
-	err = mutex_lock_killable_nested(&lo->lo_ctl_mutex, 1);
+	err = mutex_lock_killable(&lo->lo_ctl_mutex);
 	if (err)
 		goto out_unlocked;