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    SubjectRe: WARNING in close_fs_devices (2)
    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@syzkaller.appspotmail.com
    >
    > ------------[ 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.

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