Comment # 4 on bug 1047152 from Aleksa Sarai

> > I am a
> > bit  confused why this eventually causes every other btrfs operation to
> > block as well, but that's what I see.
> 
> I think it's because close_ctree also calls
> btrfs_qgroup_wait_for_completion[1].

Never mind (sorry for the spam), that's completely unrelated (close_ctree only
is called when unmounting AFAICS). Though I just noticed that open_ctree (which
looks like it's used when mounting) should clear qgroup_rescan_running using
btrfs_init_qgroup.