On 07/01/2016 08:38 PM, Tejun Heo wrote:

On Fri, Jul 01, 2016 at 12:00:50PM +0200, Jan Kara wrote:

quoted

Hello,

On Thu 30-06-16 14:18:14, Nikolay Borisov wrote:

quoted

In light of the discussion in https://patchwork.kernel.org/patch/9187411/ and 
the discussion at https://groups.google.com/forum/#!topic/syzkaller/XvxH3cBQ134

Well, it looks it is also some bdi_writeback lifetime issue but I don't see
how it would be related to I_DIRTY_TIME issues. There were couple of fixes
related to bdi_writeback issues from Tejun since 4.4. Maybe Tejun can tell
you whether he's seen this or not...

Yeah, this could be the same problem from the blkdev detach path.
Nikolay, was there any block device hot unplugging going on?  If not,
do you have a reproducer?

So the btrfs fs was created inside a loop device and mounted with -o loop. 
Evidently from the oops it seems that this is the normal umount path, meaning 
that no device hot plugging was in action. Unfortunately I don't have a reproducer. 
However, the workload looks like the following: 

1. mount -t btrfs -o compress=zlib -o subvol="${subvol}" -o loop "${storage_file}" "${mount_point}"
2. Do reads/writes (rsync) (dirtying some pages)
3. umount "${mount_point}"

Thanks.