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

[Brian Starkey <Brian.Starkey@arm.com>]

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@arm.com>
> > > ---
> > >  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".

Cheers,
-Brian

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