[PATCH] Proposed removal of IS_ERR_OR_NULL() (was: Re: [PATCH 1/4] gpiolib: introduce descriptor-based GPIO interface)

[Grant Likely]

On Wed, Jan 9, 2013 at 3:04 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> So, it seems there's some concensus building here, and it seems that
> I've become the chosen victi^wvolunteer for this.  So, here's a patch.
> It's missing a Guns-supplied-by: tag though.
>
> From: Russell King <rmk+kernel at arm.linux.org.uk>
> Subject: Mark IS_ERR_OR_NULL() deprecated
>
> IS_ERR_OR_NULL() attracts a lot of abuse: people use it without much
> thought about it's effects.  Common errors include:
>  1. checking the returned pointer for functions defined as only
>     returning errno-pointer values, rather than using IS_ERR().
>     This leads to: ptr = foo(); if (IS_ERR_OR_NULL(ptr)) return
>      PTR_ERR(ptr);
>  2. using it to check functions which only ever return NULL on error,
>     thereby leading to another zero-error value return.
> In the case of debugfs functions, these return errno-pointer values when
> debugfs is configured out, which means code which blindly checks using
> IS_ERR_OR_NULL() ends up returning errors, which is rather perverse for
> something that's not implemented.
>
> Therefore, let's schedule it for removal in a few releases.
>
> Nicolas Pitre comments:
>> I do agree with Russell here.  Despite the original intentions behind
>> IS_ERR_OR_NULL() which were certainly legitimate, the end result in
>> practice is less reliable code with increased maintenance costs.
>> Unlike other convenience macros in the kernel, this one is giving a
>> false sense of correctness with too many people falling in the trap
>> of using it just because it is available.
>>
>> I strongly think this macro should simply be removed from the source
>> tree entirely and the code reverted to explicit tests against NULL
>> when appropriate.
>
> Suggested-by: David Howells <dhowells at redhat.com>
> Tape-measuring-service-offered-by: Will Deacon <will.deacon at arm.com>
> Victim-for-firing-sqad: Russell King <rmk+kernel at arm.linux.org.uk>
> Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>

Acked-by: Grant Likely <grant.likely at secretlab.ca>

I fully agree with doing this. While I'm not a fan of the PTR_ERR
pattern, this does (hopefully) make users think just a little bit more
about it.

> ---
> Ok, so I'm in the firing line for suggesting this, but it appears
> several people wish this to happen.
>
> I'm not intending to push this patch forwards _just_ yet: we need to
> sort out the existing users _first_ to prevent the kernel turning into
> one hell of a mess of warnings.

I currently see 355 users. That's a lot, but not inconceivable for an
auditing effort for pulling them out instead of scheduling for future
removal.

g.