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Message-ID: <20181015093708.GA22876@mail.gnudd.com>
Date: Mon, 15 Oct 2018 11:37:08 +0200
From: Alessandro Rubini <rubini@...dd.com>
To: uwe@...ine-koenig.org
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.
> 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
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