Re: WARNING in close_fs_devices (2)

[David Sterba]

On Fri, Sep 18, 2020 at 04:22:16AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    e4c26faa Merge tag 'usb-5.9-rc5' of git://git.kernel.org/p..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15bf1621900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c61610091f4ca8c4
> dashboard link: https://syzkaller.appspot.com/bug?extid=4cfe71a4da060be47502
> compiler:       gcc (GCC) 10.1.0-syz 20200507
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+4cfe71a4da060be47502@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 3612 at fs/btrfs/volumes.c:1166 close_fs_devices.part.0+0x607/0x800 fs/btrfs/volumes.c:1166

1152 static int close_fs_devices(struct btrfs_fs_devices *fs_devices)
1153 {
1154         struct btrfs_device *device, *tmp;
1155
1156         if (--fs_devices->opened > 0)
1157                 return 0;
1158
1159         mutex_lock(&fs_devices->device_list_mutex);
1160         list_for_each_entry_safe(device, tmp, &fs_devices->devices, dev_list) {
1161                 btrfs_close_one_device(device);
1162         }
1163         mutex_unlock(&fs_devices->device_list_mutex);
1164
1165         WARN_ON(fs_devices->open_devices);
1166         WARN_ON(fs_devices->rw_devices);

1167         fs_devices->opened = 0;
1168         fs_devices->seeding = false;
1169
1170         return 0;
1171 }

It's the 2nd warning, rw_devices > 0 .

> Kernel panic - not syncing: panic_on_warn set ...
> CPU: 1 PID: 3612 Comm: syz-executor.2 Not tainted 5.9.0-rc4-syzkaller #0

5.9-rc4, we've had some changes around device locking but no idea which
one it could be.