[PATCH 07/22] drm/tegra: Remove module ownership from the tegra_fb_ops

[Thierry Reding]

On Tue, Jun 13, 2017 at 05:00:28PM +0300, Dmitry Osipenko wrote:
> On 13.06.2017 16:43, Thierry Reding wrote:
> > On Tue, May 23, 2017 at 03:14:22AM +0300, Dmitry Osipenko wrote:
> >> The framebuffers console fbcon_startup() increments the tegra_drm module
> >> 'use' refcount via try_module_get(), causing an interlock of the DRM subsys
> >> and the tegra_drm modules. In result, the tegra_drm module can't be unloaded
> >> using rmmod.
> >>
> >> Signed-off-by: Dmitry Osipenko <digetx at gmail.com>
> >> ---
> >>  drivers/gpu/drm/tegra/fb.c | 1 -
> >>  1 file changed, 1 deletion(-)
> > 
> > That's done on purpose because otherwise you could just rip out the
> > driver from under the framebuffer emulation and things would crash.
> > 
> > My understanding is that the right way to unload a module is to unbind
> > the driver first (which will cause the framebuffer to be removed and
> > hence the reference to be dropped) before the rmmod.
> > 
> > Thierry
> > 
> 
> Aha, interesting. I'll try the unbinding and will drop this patch from the
> series, thank you for the clarification. I haven't observed any crashes on a
> module reloading (framebuffer detached/attached just fine), maybe I was lucky then.

It's possible that it works by accident. The driver removes the
framebuffer as part of the driver removal process, so technically
nothing should crash. However, if, for any reason, anyone was holding
on to a reference to the framebuffer (not sure if that is even possible)
the module needs to stay around long enough as well, otherwise the
function pointers would become dangling. The module reference makes sure
that this doesn't happen (as long as a framebuffer exists, the ops will
stick around).

So even if there isn't a way to make it crash today, the code is still
correct in taking the reference.

Thierry
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