linux-kernel - Re: [PATCH RFC] err.h: document that PTR_ERR should only be used if IS

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Date:   Mon, 15 Oct 2018 11:46:21 +0200
From:   Uwe Kleine-König <uwe@...ine-koenig.org>
To:     Alessandro Rubini <rubini@...dd.com>
Cc:     gregkh@...uxfoundation.org, corbet@....net,
        linux-kernel@...r.kernel.org, thierry.reding@...il.com,
        Michal.Vokac@...ft.com, akpm@...ux-foundation.org,
        kernel@...gutronix.de
Subject: Re: [PATCH RFC] err.h: document that PTR_ERR should only be used if
 IS_ERR returns true

Hello,

On 10/15/2018 11:37 AM, Alessandro Rubini wrote:
>> during a review I claimed that PTR_ERR should only be used if IS_ERR was
>> already checked. The rationale isn't obvious though and Thierry
>> suggested to keep the code as is and not introduce an IS_ERR check.
> 
> The rationale is the same ch11 you linked to: "any other value
> is a valid pointer".  It isn't usefult to convert to long sth that
> your are not using as a long.  You should not pass it to strerror(-err)
> for example.

ok, that's obvious that this should be forbidden.

> OTOH I admit you can compare any value with -EINVAL, after PTR_ERR.
> But in general you first detect the error condition and then split
> among error (or print a message according to the exact value.
> 
>> maybe something like "On an Alpha it is important because
>> not doing it results in a bus error there."
> 
> No, nothing that exotic.

OK, if there is nothing that exotic, the patch is probably of little use.

> You said:
> 
>> Thierry suggested to keep the code as is and not introduce an IS_ERR check.
> 
> I wonder where. Sure no extra check in the header, that would be
> extra wasted time in every caller. If it's a specific caller place,
> it may make sense to avoid the check, I don't know the details.

http://patchwork.ozlabs.org/patch/981774/#2009383

The obvious alternatives would be:

	if (PTR_ERR_OR_ZERO(imx_chip->pwm_gpiod) == -EPROBE_DEFER)

	if (imx_chip->pwm_gpiod == ERR_PTR(-EPROBE_DEFER))

but if no kittens die anywhere it's probably of little value to argue here.

> As for the specific patch you propose, I'm unsure it's useful.  Maybe
> we should remember that "this returns the equivalent of "-errno" if
> IS_ERR() is true", but I'm personally not much for overcommenting:
> It's a simple cast and there are a zillion users to see how exactly
> this works if anyone is uncertain.

ack.

Thanks for your input
Uwe



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