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Subject: Re: [PATCH] btrfs: fix rw device counting in
 __btrfs_free_extra_devids
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>, clm@fb.com,
        josef@toxicpanda.com, dsterba@suse.com
Cc:     anand.jain@oracle.com, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org, skhan@linuxfoundation.org,
        gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
References: <20210715103403.176695-1-desmondcheongzx@gmail.com>
From:   Nikolay Borisov <nborisov@suse.com>
Message-ID: <7ae7a858-9893-c41c-ed96-10651c295087@suse.com>
Date:   Thu, 15 Jul 2021 14:55:37 +0300
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On 15.07.21 г. 13:34, Desmond Cheong Zhi Xi wrote:
> Syzbot reports a warning in close_fs_devices that happens because
> fs_devices->rw_devices is not 0 after calling btrfs_close_one_device
> on each device.
> 
> This happens when a writeable device is removed in
> __btrfs_free_extra_devids, but the rw device count is not decremented
> accordingly. So when close_fs_devices is called, the removed device is
> still counted and we get an off by 1 error.
> 
> Here is one call trace that was observed:
>   btrfs_mount_root():
>     btrfs_scan_one_device():
>       device_list_add();   <---------------- device added
>     btrfs_open_devices():
>       open_fs_devices():
>         btrfs_open_one_device();   <-------- rw device count ++
>     btrfs_fill_super():
>       open_ctree():
>         btrfs_free_extra_devids():
> 	  __btrfs_free_extra_devids();  <--- device removed
> 	  fail_tree_roots:
> 	    btrfs_close_devices():
> 	      close_fs_devices();   <------- rw device count off by 1
> 
> Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")
> Reported-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
> Tested-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

Is there a reliable reproducer from syzbot? Can this be turned into an
xfstest?

> ---
>  fs/btrfs/volumes.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 807502cd6510..916c25371658 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
>  		if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
>  			list_del_init(&device->dev_alloc_list);
>  			clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
> +			fs_devices->rw_devices--;
>  		}
>  		list_del_init(&device->dev_list);
>  		fs_devices->num_devices--;
>