linux-kernel - Re: [PATCH 3/4] clk/Renesas-MSTP: Less function calls in cpg_mstp_clocks

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Date:   Thu, 15 Sep 2016 22:40:22 +0200
From:   SF Markus Elfring <elfring@...rs.sourceforge.net>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     linux-clk <linux-clk@...r.kernel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Michael Turquette <mturquette@...libre.com>,
        Simon Horman <horms+renesas@...ge.net.au>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        Julia Lawall <julia.lawall@...6.fr>
Subject: Re: [PATCH 3/4] clk/Renesas-MSTP: Less function calls in
 cpg_mstp_clocks_init() after error detection

> It's perfectly legal to call kfree() on a NULL pointer.

I know this function property well.


>> * Split a condition check for memory allocation failures so that
>>   each pointer from these function calls will be checked immediately.
>>
>>   See also background information:
>>   Topic "CWE-754: Improper check for unusual or exceptional conditions"
>>   Link: https://cwe.mitre.org/data/definitions/754.html
>>
>> * Return directly after a call of the function "kzalloc" failed
>>   at the beginning.
> 
> Both calls are already close together.

Can it be that an other software development concern is eventually
overlooked because of this "neighbourship" (or is categorised with
a lower priority)?

I suggest to reconsider this design detail if it is really acceptable
for the safe implementation of such a software module.

* How much will it matter in general that one function call was performed
  in this use case without checking its return values immediately?

* Should it usually be determined quicker if a required resource like
  memory could be acquired before trying the next allocation?


> In addition, your patch increases the LoC, IMHO without improving the code.

I find this consequence still debatable.


>> diff --git a/drivers/clk/renesas/clk-mstp.c b/drivers/clk/renesas/clk-mstp.c
>> index 1fdc44b..6c82e0e 100644
>> --- a/drivers/clk/renesas/clk-mstp.c
>> +++ b/drivers/clk/renesas/clk-mstp.c
>> @@ -167,10 +167,12 @@ static void __init cpg_mstp_clocks_init(struct device_node *np)
>>         unsigned int i;
>>
>>         group = kzalloc(sizeof(*group), GFP_KERNEL);
>> +       if (!group)
>> +               return;
>> +
>>         clks = kmalloc_array(MSTP_MAX_CLOCKS, sizeof(*clks), GFP_KERNEL);
>> -       if (group == NULL || clks == NULL) {
>> +       if (!clks) {
>>                 kfree(group);
>> -               kfree(clks);
>>                 return;
>>         }

Is this update suggestion worth for another look?

Regards,
Markus