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

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Date:   Fri, 30 Dec 2022 12:03:25 +0200
From:   Mikko Perttunen <cyndis@...si.fi>
To:     Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>
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 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.

Mikko

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