Re: [PATCH v2 1/2] drm/drm_crtc: ensure dma_fence_ops remain valid during device unbind

["André Draszik" <andre.draszik@linaro.org>]

Hi Philipp,

On Thu, 2026-07-09 at 14:32 +0200, Philipp Stanner wrote:
> +Cc Danilo (who is currently concerned with drm_device life times)
> 
> On Wed, 2026-07-08 at 16:22 +0100, André Draszik wrote:
> > 
> 
> […]
> 
> > Link: https://sashiko.dev/#/patchset/20260618-linux-drm_crtc_fix2-v1-1-c03e77b36f34@linaro.org?part=1
> > Signed-off-by: André Draszik <andre.draszik@linaro.org>
> 
> I am tempted to think that this also needs a Fixes and needs to be
> backported into stable kernels, doesn't it? Especially if the BUG_ON
> disappears in stable kernels.

Good point, thanks. I forgot to add this in and will try to find a reasonable
commit to relate to.

> 
> > ---
> >  drivers/gpu/drm/drm_crtc.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> > index 63ead8ba6756..d55f1377ec36 100644
> > --- a/drivers/gpu/drm/drm_crtc.c
> > +++ b/drivers/gpu/drm/drm_crtc.c
> > @@ -501,6 +501,12 @@ void drm_crtc_cleanup(struct drm_crtc *crtc)
> >  {
> >  	struct drm_device *dev = crtc->dev;
> >  
> > +	/* Ensure our dma_fence_ops remain valid for an RCU grace period after
> > +	 * the fence is signaled. This is necessary because our dma_fence_ops
> > +	 * dereference crtc->dev.
> > +	 */
> > +	synchronize_rcu();
> 
> nit:
> I guess this is the only place where one can reasonably put the
> synchronize_rcu(). But I would hint at the RCU delay in the function's
> docu.

Unfortunately, this still looks like an incomplete fix -
https://sashiko.dev/#/patchset/20260618-linux-drm_crtc_fix2-v1-1-c03e77b36f34@linaro.org?part=1

My next version will simply copy the relevant strings into a custom

struct drm_crtc_fence {
	struct dma_fence base;
	char driver_name[32];
	char timeline_name[32];
};

or similar as part of drm_crtc_create_fence() and just use those as part
of the dma_fence_ops. That approach should avoid all race conditions and
corner cases with RCU.

I'll also make sure to update relevant documentation.


Cheers,
Andre'