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

From: Alessandro Rubini <rubini@gnudd.com>
To: uwe@kleine-koenig.org
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 11:37:08 +0200	[thread overview]
Message-ID: <20181015093708.GA22876@mail.gnudd.com> (raw)
In-Reply-To: <20181014202807.16412-1-uwe@kleine-koenig.org>

Hello.

> 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.

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.

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.

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.

Regards
/alessandro