[PATCH v2] drm: Do not call drm_dev_unregister twice on drm_unplug_dev

[Chris Wilson]

On Sun, May 28, 2017 at 07:16:55PM +0200, Hans de Goede wrote:
> Since commit a39be606f99d ("drm: Do a full device unregister when
> unplugging") drm_unplug_dev has been calling drm_dev_unregister followed
> by a drm_put_dev when open_count reaches 0. This drm_put_dev calls
> drm_dev_unregister again. Since drm_dev_unregister is not protected
> against being called multiple times this leads to havoc.
> 
> This commit fixes this by calling drm_dev_unref instead of drm_put_dev.
> 
> Fixes: a39be606f99d ("drm: Do a full device unregister when unplugging")
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Marco Diego Aurélio Mesquita <marcodiegomesquita at gmail.com>
> Reported-by: Marco Diego Aurélio Mesquita <marcodiegomesquita at gmail.com>
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
> Note I don't have any USB display devices at hand for testing atm so
> this patch has only been compile tested.

Not tested it either, but yes we do end up calling
drm_device_unregister() twice and indeed that looks bad.

This morning I was thinking about kselftests to simulate the different
conditions and avoid us having to get our hands dirty with real hw. It
also reminded me of https://patchwork.kernel.org/patch/9678823/ which
has the tantalising prospect of removing drm_unplug_dev() entirely.

Another benefit is the removal of one more call to drm_put_dev() which
has been deprecated for a while.

The patch definitely fixes a bug at first glance, but I wonder if it is
really telling me that the call to drm_device_unregister() here is the
fundamental issue. But I am short of actual answers. 
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre