Re: [PATCH v1 2/2] drm: Clear the fence pointer when writeback job signaled

[james qian wang (Arm Technology China)]

On Fri, Aug 02, 2019 at 05:29:20PM +0800, Brian Starkey wrote:
> Hi Lowry,
> 
> On Thu, Aug 01, 2019 at 06:34:08AM +0000, Lowry Li (Arm Technology China) wrote:
> > Hi Brian,
> > 
> > On Wed, Jul 31, 2019 at 09:20:04PM +0800, Brian Starkey wrote:
> > > Hi Lowry,
> > > 
> > > Thanks for this cleanup.
> > > 
> > > On Wed, Jul 31, 2019 at 11:04:45AM +0000, Lowry Li (Arm Technology China) wrote:
> > > > During it signals the completion of a writeback job, after releasing
> > > > the out_fence, we'd clear the pointer.
> > > > 
> > > > Check if fence left over in drm_writeback_cleanup_job(), release it.
> > > > 
> > > > Signed-off-by: Lowry Li (Arm Technology China) <lowry.li@xxxxxxx>
> > > > ---
> > > >  drivers/gpu/drm/drm_writeback.c | 23 +++++++++++++++--------
> > > >  1 file changed, 15 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
> > > > index ff138b6..43d9e3b 100644
> > > > --- a/drivers/gpu/drm/drm_writeback.c
> > > > +++ b/drivers/gpu/drm/drm_writeback.c
> > > > @@ -324,6 +324,9 @@ void drm_writeback_cleanup_job(struct drm_writeback_job *job)
> > > >  	if (job->fb)
> > > >  		drm_framebuffer_put(job->fb);
> > > >  
> > > > +	if (job->out_fence)
> > > 
> > > I'm thinking it might be a good idea to signal the fence with an error
> > > here, if it's not already signaled. Otherwise, if there's someone
> > > waiting (which there shouldn't be), they're going to be waiting a very
> > > long time :-)
> > > 
> > > Thanks,
> > > -Brian
> > > 
> > Here it happened at atomic_check failed and test only commit. For both
> > cases, the commit has been dropped and it's only a clean up. So here better
> > not be treated as an error case:)
> 
> If anyone else has a reference on the fence, then IMO it absolutely is
> an error to reach this point without the fence being signaled -
> because it means that the fence will never be signaled.
> 
> I don't think the API gives you a way to check if this is the last
> reference, so it's safest to just make sure the fence is signalled
> before dropping the reference.
> 
> It just feels wrong to me to have the possibility of a dangling fence
> which is never going to get signalled; and it's an easy defensive step
> to make sure it can never happen.
> 
> I know it _shouldn't_ happen, but we often put in handling for cases
> which shouldn't happen, because they frequently do happen :-)
> 
> > 
> > Since for userspace, it should have been failed or a test only case, so
> > writebace fence should not be signaled.
> 
> It's not only userspace that can wait on fences (and in fact this
> fence will never even reach userspace if the commit fails), the driver
> may have taken a copy to use for "something".
>

Maybe we can add a wb_fence canceling into complete_signaling() for the
atomic_check failed cleanup like the crtc->out_fence.

Then if in this place we still can got a fence here, that must be a
error we signal and WARN it

Thanks
James

> Cheers,
> -Brian
> 
> > 
> > Best regards,
> > Lowry
> > > > +		dma_fence_put(job->out_fence);
> > > > +
> > > >  	kfree(job);
> > > >  }
> > 
> > -- 
> > Regards
> > Lowry