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

[Li, Yunxiang (Teddy)]

[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 This patchset is outdated by now and I've sent a fix to amd-gfx for the specific issue in amdgpu.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.

Yunxiang