Re: [PATCH] drm/tegra: submit: No need for Null pointer check before kfree

[Deepak R Varma]

On Wed, Dec 28, 2022 at 03:17:59PM +0200, Mikko Perttunen wrote:
> On 12/28/22 15:08, Deepak R Varma wrote:
> > On Wed, Dec 28, 2022 at 02:28:54PM +0200, Mikko Perttunen wrote:
> > > On 12/27/22 19:14, Deepak R Varma wrote:
> > > > kfree() & vfree() internally perform NULL check on the pointer handed
> > > > to it and take no action if it indeed is NULL. Hence there is no need
> > > > for a pre-check of the memory pointer before handing it to
> > > > kfree()/vfree().
> > > >
> > > > Issue reported by ifnullfree.cocci Coccinelle semantic patch script.
> > > >
> > > > Signed-off-by: Deepak R Varma <drv@xxxxxxxxx>
> > > > ---
> > > >    drivers/gpu/drm/tegra/submit.c | 4 ++--
> > > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/tegra/submit.c b/drivers/gpu/drm/tegra/submit.c
> > > > index 066f88564169..06f836db99d0 100644
> > > > --- a/drivers/gpu/drm/tegra/submit.c
> > > > +++ b/drivers/gpu/drm/tegra/submit.c
> > > > @@ -680,8 +680,8 @@ int tegra_drm_ioctl_channel_submit(struct drm_device *drm, void *data,
> > > >    		kfree(job_data->used_mappings);
> > > >    	}
> > > >
> > > > -	if (job_data)
> > > > -		kfree(job_data);
> > > > +	kfree(job_data);
> > > > +
> > > >    put_bo:
> > > >    	gather_bo_put(&bo->base);
> > > >    unlock:
> > > > --
> > > > 2.34.1
> > > >
> > > >
> > > >
> > >
> > > It continues to be the case that I think this transform is bad. Same applies
> > > to the host1x patch.
> >
> > Hello Mikko,
> > Thank you for responding to the patch proposal. Could you please explain why is
> > this bad?
> >
> > Regards,
> > ./drv
> >
> > >
> > > Mikko
> >
> >
>
> Hi,
>
> it gets rid of visual hints on code paths indicating the possible liveness
> of pointer variables. I.e., after the change, whether the pointer can be
> NULL or not is more difficult to reason about locally, instead requiring
> more global reasoning which is mentally more taxing.
>
> Since C's type system doesn't help with tracking these kinds of things, I
> believe it is important to have these kinds of local contextual cues to help
> the programmer.

Hello Mikko,
That really helps. Thank you for the detailed explanation. I do have an extended
question though. In this context, when we are ready to release the memory, how
is it useful to know if it is NULL or not this late in the flow when the scope
is about to end?

Thanks again!
./drv




>
> Mikko