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

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Message-ID: <Y7MgdKD14kLwOo0S@qemulion>
Date:   Mon, 2 Jan 2023 23:50:36 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Mikko Perttunen <cyndis@...si.fi>
Cc:     Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.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, drv@...lo.com
Subject: Re: [PATCH] drm/tegra: submit: No need for Null pointer check before
 kfree

On Fri, Dec 30, 2022 at 12:03:25PM +0200, Mikko Perttunen wrote:
> On 12/30/22 12:01, Mikko Perttunen wrote:
> > On 12/30/22 11:15, Stanislaw Gruszka wrote:
> > > 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
> >
> > It can be NULL; see:
> >
> >          job->user_data = job_data;
> >          job->release = release_job;
> >          job->timeout = 10000;
> >
> >          /*
> >           * job_data is now part of job reference counting, so don't
> > release
> >           * it from here.
> >           */
> >          job_data = NULL;
> >
> > If we go into free_job_data after this code (which happens if there is
> > no error, or if host1x_job_submit fails), job_data will be NULL.
> >
> > The memory is instead released in the 'put_job' label; host1x_job_put
> > ends up calling release_job, which does the kfree.
>
> Well, the refcount is dropped -- it's not necessarily freed immediately, if
> the job is in execution.

Thanks Mikko. I Agree. Hence I think there is no change for the program at
runtime. The proposed change looks safe to me.

./drv

>
> Mikko
>
> >
> > (Yes, it is rather complicated..)
> >
> > Thanks,
> > Mikko
>