linux-kernel - Re: [PATCH][RFC] kernel.h: provide array iterator

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Date:   Sat, 17 Mar 2018 10:32:57 +0100
From:   Kieran Bingham <kieran.bingham@...asonboard.com>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        linux-kernel@...r.kernel.org
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        linux-media@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ian Abbott <abbotti@....co.uk>
Subject: Re: [PATCH][RFC] kernel.h: provide array iterator

Hi Rasmus,

On 16/03/18 16:27, Rasmus Villemoes wrote:
> On 2018-03-15 11:00, Kieran Bingham wrote:
>> Simplify array iteration with a helper to iterate each entry in an array.
>> Utilise the existing ARRAY_SIZE macro to identify the length of the array
>> and pointer arithmetic to process each item as a for loop.
>>
>> Signed-off-by: Kieran Bingham <kieran.bingham@...asonboard.com>
>> ---
>>  include/linux/kernel.h | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> The use of static arrays to store data is a common use case throughout the
>> kernel. Along with that is the obvious need to iterate that data.
>>
>> In fact there are just shy of 5000 instances of iterating a static array:
>> 	git grep "for .*ARRAY_SIZE" | wc -l
>> 	4943
>>
>> When working on the UVC driver - I found that I needed to split one such
>> iteration into two parts, and at the same time felt that this could be
>> refactored to be cleaner / easier to read. 
> 
> About that, it would be helpful if you first converted to the new
> iterator, so that one can more easily see they are equivalent. And then
> split in two, adding the flush_workqueue call. Or do it the other way
> around. But please don't mix the two in one patch, especially not if
> it's supposed to act as an example of how to use the new helper.
>

My apologies - in the example below I was trying to show the usage and reason
for the macro. This was not meant to be a change to be integrated - the 'example
change' is not how the change will be committed, or included in the patch - but
was added here purely to show the usage / reason for the new macro and promote
the discussion. (So I'll already call that a success)

But that is a good point - the example usage could be much simplified here, and
then included in the commit message.


>> I do however worry that this simple short patch might not be desired or could
>> also be heavily bikeshedded due to it's potential wide spread use (though
>> perhaps that would be a good thing to have more users) ...  but here it is,
>> along with an example usage below which is part of a separate series.
> 
> I think it can be useful, and it does have the must_be_array protection
> built in, so code doesn't silently break if one changes from a
> fixed-size allocation to e.g. a kmalloc-based one. Just don't attempt a
> tree-wide mass conversion, but obviously starting to make use of it when
> refactoring code anyway is fine.

Well it had already been suggested to try to make a coccinelle patch - but I
suspect time and effort required may delay or postpone that currently.

I'll focus on seeing if I can actually get this macro in before expending effort
on a full conversion :-D

I originally anticipated that this would be a 'convert or use as required' style
change.


> And now, the bikeshedding you expected :)

It wouldn't be a discussion without it :D


>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>> index ce51455e2adf..95d7dae248b7 100644
>> --- a/include/linux/kernel.h
>> +++ b/include/linux/kernel.h
>> @@ -70,6 +70,16 @@
>>   */
>>  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
>>  
>> +/**
>> + * for_each_array_element - Iterate all items in an array
>> + * @elem: pointer of array type for iteration cursor
> 
> Hm, "pointer of array type" sounds wrong; it's not a "pointer to array".
> But "pointer of array elements' type" is clumsy. Maybe just "@elem:
> iteration cursor" is clear enough.

"@elem: iteration cursor" sounds good to me.

Depending on how the other conversations go here - I will likely make this
change. (I see there was a previous attempt at including a very similar macro)


>> + * @array: array to be iterated
>> + */
>> +#define for_each_array_element(elem, array) \
>> +	for (elem = &(array)[0]; \
>> +	     elem < &(array)[ARRAY_SIZE(array)]; \
>> +	     ++elem)
>> +
> 
> Please parenthesize elem as well.

That's certainly a good point! Thanks :D

> Rasmus

Regards

Kieran Bingham