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

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Date:   Fri, 30 Dec 2022 10:15:01 +0100
From:   Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>
To:     Mikko Perttunen <cyndis@...si.fi>
Cc:     Deepak R Varma <drv@...lo.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Jonathan Hunter <jonathanh@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH] drm/tegra: submit: No need for Null pointer check before
 kfree

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@...lo.com>
> > > > ---
> > > >    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.

I agree with your point of view. But regarding this particular patch,
at least on code base I can see, after free_job_data label job_done
can not be NULL. So patch seems to be ok, but maybe changelog need to
be different

Regards
Stanislaw