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

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Message-ID: <Y6xQO+w/P+m8w6ke@qemulion>
Date:   Wed, 28 Dec 2022 19:48:35 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Mikko Perttunen <cyndis@...si.fi>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Hunter <jonathanh@...dia.com>,
        dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH] drm/tegra: submit: No need for Null pointer check before
 kfree

On Wed, Dec 28, 2022 at 03:48:05PM +0200, Mikko Perttunen wrote:
> On 12/28/22 15:34, Deepak R Varma wrote:
> > On Wed, Dec 28, 2022 at 03:17:59PM +0200, Mikko Perttunen wrote:
> > > On 12/28/22 15:08, Deepak R Varma wrote:
> > >
> > > 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?
>
> In the current code it doesn't matter, but if someone went to change this
> code (for example to add another release step), and we just had
> 'kfree(job_data)', they would have to remember that kfree works with NULL
> pointers, and would have to go looking elsewhere in the code to see if it is
> in fact possible to assume that job_data cannot be NULL here, or not. If
> they forget about kfree working with NULL pointers, which wouldn't be that
> surprising since it is almost always only called with non-NULL pointers,
> they might instead introduce a bug.
>
> In this particular instance it's probably not that bad since immediately
> above we have another 'if' block that checks if job_data is NULL, which
> serves as a hint to the programmer; however, as a general principle it
> stands that having the NULL check here makes it obvious to any reading
> programmer that they any changes they make have to consider if the pointer
> is NULL or not.

Sounds good. Thanks again. Would like to see if other experts have any further
guidance on this patch.

Regards,
./drv

>
> >
> > > Mikko
> >
> >
>