[PATCH 2/2] drm: get lock before accessing vblank refcount

[Daniel Vetter]

On Tue, Sep 06, 2022 at 08:18:30PM +0000, Li, Yunxiang (Teddy) wrote:
> [Public]
> 
> Hi Daniel,
> 
> I added the check because I saw that the refcount was always
> updated/read with lock held elsewhere, and the pattern looked very
> similar to in the put e.g. drm_crtc_vblank_reset. This patchset is
> outdated by now and I've sent a fix to amd-gfx for the specific issue in
> amdgpu.
> 
> However, I think the way drm_crtc_vblank_on/off functions increments the
> refcount without enabling the vblank is still a bit risky given how many
> unchecked calls to drm_vblank_get there is elsewhere. Maybe it's more
> appropriate to simply add an WARN to drm_vblank_get when it's called
> with inmodeset set? This way the WARN happens right at the problematic
> point, instead of far into the future when the put is called.

drm_crtc_vblank_get failing when the crtc is off is how this is supposed
to work, calling WARN_ON or similar in there would upset everything.

What might be an option is adding __must_check or similar annotations, but
the problem is that in many cases the driver knows that it cannot fail, so
this isn't great either.

Another option would be to split this up into drm_crtc_vblank_get with
void return value (and a WARN_ON when it fails), and
drm_crtc_vblank_try_get, which can fail. And then go through _all_ the
callers and audit them.

Imo not really worth the work, but we could do that.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch