Re: [PATCH RFC] err.h: document that PTR_ERR should only be used if IS_ERR returns true - Uwe Kleine-König

From: "Uwe Kleine-König" <uwe@kleine-koenig.org>
To: Alessandro Rubini <rubini@gnudd.com>
Cc: gregkh@linuxfoundation.org, corbet@lwn.net,
	linux-kernel@vger.kernel.org, thierry.reding@gmail.com,
	Michal.Vokac@ysoft.com, akpm@linux-foundation.org,
	kernel@pengutronix.de
Subject: Re: [PATCH RFC] err.h: document that PTR_ERR should only be used if IS_ERR returns true
Date: Mon, 15 Oct 2018 11:46:21 +0200	[thread overview]
Message-ID: <41de7e1a-1c93-a27d-b777-7e4db4e8d11b@kleine-koenig.org> (raw)
In-Reply-To: <20181015093708.GA22876@mail.gnudd.com>

[-- Attachment #1.1: Type: text/plain, Size: 1954 bytes --]

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

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]