Re: [PATCH] btrfs: fix rw device counting in __btrfs_free_extra_devids

[Nikolay Borisov]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Tested-by: syzbot+a70e2ad0879f160b9217@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>

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--;
>