linux-kernel - Re: [PATCH 03/21] ata: libahci_platform: Explicitly set rc on devres

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Message-ID: <f163f982-852f-027f-8d24-e624e45fbbf1@opensource.wdc.com>
Date:   Fri, 25 Mar 2022 10:56:29 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Hans de Goede <hdegoede@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Rob Herring <robh+dt@...nel.org>, linux-ide@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 03/21] ata: libahci_platform: Explicitly set rc on
 devres_alloc failure

On 2022/03/25 6:37, Serge Semin wrote:
> Hello Damien
> 
> On Thu, Mar 24, 2022 at 09:58:34AM +0900, Damien Le Moal wrote:
>> On 3/24/22 09:16, Serge Semin wrote:
>>> It's better for readability and maintainability to explicitly assign an
>>> error number to the variable used then as a return value from the method
>>> on the cleanup-on-error path. So adding new code in the method we won't
>>
> 
>> No it is not. On-stack variable initialization is not free. So if
>> initializing the variable is not needed, do not do it.
> 
> This patch isn't about on-stack initialization, but about bringing an
> order to the error-handling procedure of the
> ahci_platform_get_resources() method. Explicitly setting the rc variable
> with an error value closer to the place caused the error much easier
> to perceive than keeping in mind that the variable has been set with
> some default value. That turns to be even more justified seeing the
> rest of the method does it that way.

I agree with that change. Setting "rc = -ENOMEM" under the "if" checking for
error is fine.

> 
> See my next comment regarding the initialization.
> 
>>
>>> have to think whether the overridden rc-variable is set afterward in case
>>> of an error. Saving one line of code doesn't worth it especially seeing
>>> the rest of the ahci_platform_get_resources() function errors handling
>>> blocks do explicitly write errno to rc.
>>>
>>> Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
>>> ---
>>>  drivers/ata/libahci_platform.c | 6 ++++--
>>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/ata/libahci_platform.c b/drivers/ata/libahci_platform.c
>>> index 18296443ccba..1bd2f1686239 100644
>>> --- a/drivers/ata/libahci_platform.c
>>> +++ b/drivers/ata/libahci_platform.c
>>> @@ -389,7 +389,7 @@ struct ahci_host_priv *ahci_platform_get_resources(struct platform_device *pdev,
>>>  	struct ahci_host_priv *hpriv;
>>>  	struct clk *clk;
>>>  	struct device_node *child;
>>> -	int i, enabled_ports = 0, rc = -ENOMEM, child_nodes;
>>> +	int i, enabled_ports = 0, rc = 0, child_nodes;
>>>  	u32 mask_port_map = 0;
>>>  
>>>  	if (!devres_open_group(dev, NULL, GFP_KERNEL))
>>> @@ -397,8 +397,10 @@ struct ahci_host_priv *ahci_platform_get_resources(struct platform_device *pdev,
>>>  
>>>  	hpriv = devres_alloc(ahci_platform_put_resources, sizeof(*hpriv),
>>>  			     GFP_KERNEL);
>>> -	if (!hpriv)
>>> +	if (!hpriv) {
>>> +		rc = -ENOMEM;
>>>  		goto err_out;
>>> +	}
>>
> 
>> If you set rc to -ENOMEM here, then the 0 initialization of rc is not needed.
> 
> Normally you are right. But the case of the rc/ret/etc variables is
> special. I'd stick with having it defaulted to 0 here. Here is why.
> 
> When it comes to using the rc/ret/etc variables the maintainability
> gets to be more important than some small optimization (especially
> here seeing the ahci_platform_get_resources() is called once per
> device life-time) because in case of the method alteration these
> variables very often get to be involved in one way or another. If due
> to a mistake the rc/ret/etc variable isn't updated in case of an
> erroneous situation but the method is terminated with the goto-pattern
> and rc/ret/etc isn't initialized with any default value then we will
> end up with having a garbage pointer returned. We'd be lucky if it was
> a null pointer, but in general it can be a reference to some random
> memory region. In the later case the kernel may experience random
> crashes with hard-to-find cause of the problem. In the former case the
> problem would have been tracked right away on the testing stage by
> getting the system invalid-pointer de-reference crash. That's why
> defaulting the variable to zero here is still useful.

No it is not. We have code reviews and testing to catch mistakes. If we start
thinking along these lines, any useless local variable initialization can be
justified. So let's not go there please.


-- 
Damien Le Moal
Western Digital Research