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Date:   Fri, 20 Aug 2021 12:58:28 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc:     dsterba@suse.cz, clm@fb.com, josef@toxicpanda.com,
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        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
Subject: Re: [PATCH v2] btrfs: fix rw device counting in
 __btrfs_free_extra_devids
Message-ID: <20210820105828.GN5047@twin.jikos.cz>
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On Fri, Aug 20, 2021 at 11:09:05AM +0800, Desmond Cheong Zhi Xi wrote:
> On 20/8/21 1:34 am, David Sterba wrote:
> > On Fri, Aug 20, 2021 at 01:11:58AM +0800, Desmond Cheong Zhi Xi wrote:
> >>>>> The option #2 does not sound safe because the TGT bit is checked in
> >>>>> several places where device list is queried for various reasons, even
> >>>>> without a mounted filesystem.
> >>>>>
> >>>>> Removing the assertion makes more sense but I'm still not convinced that
> >>>>> the this is expected/allowed state of a closed device.
> >>>>>
> >>>>
> >>>> Would it be better if we cleared the REPLACE_TGT bit only when closing
> >>>> the device where device->devid == BTRFS_DEV_REPLACE_DEVID?
> >>>>
> >>>> The first conditional in btrfs_close_one_device assumes that we can come
> >>>> across such a device. If we come across it, we should properly reset it.
> >>>>
> >>>> If other devices has this bit set, the ASSERT will still catch it and
> >>>> let us know something is wrong.
> >>>
> >>> That sounds great.
> >>>
> >>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> >>>> index 70f94b75f25a..a5afebb78ecf 100644
> >>>> --- a/fs/btrfs/volumes.c
> >>>> +++ b/fs/btrfs/volumes.c
> >>>> @@ -1130,6 +1130,9 @@ static void btrfs_close_one_device(struct btrfs_device *device)
> >>>>                    fs_devices->rw_devices--;
> >>>>            }
> >>>>     
> >>>> +       if (device->devid == BTRFS_DEV_REPLACE_DEVID)
> >>>> +               clear_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state);
> >>>> +
> >>>>            if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
> >>>>                    fs_devices->missing_devices--;
> >>>
> >>> I'll do a few test rounds, thanks.
> >>
> >> Just following up. Did that resolve the issue or is further
> >> investigation needed?
> > 
> > The fix seems to work, I haven't seen the assertion fail anymore,
> > incidentally the crash also stopped to show up on an unpatched branch.
> > 
> 
> Sounds good, thanks for the update. If there's anything else I can help 
> with, please let me know.

So are you going to send the patch with the fix?