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SubjectRe: [PATCH] btrfs: fix rw device counting in __btrfs_free_extra_devids
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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--;
>

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