netdev - Re: [PATCH 2/5] drivers/net/ethernet/dec/tulip: Use standard __set_bit

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Message-ID: <CAC5umyjkdDqavCo3Dk+WOggCnH+_CZz5jrOr3SougS4HSgV3OA@mail.gmail.com>
Date:	Thu, 14 Jun 2012 18:36:42 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
Cc:	Grant Grundler <grantgrundler@...il.com>,
	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>,
	akpm@...ux-foundation.org, bhutchings@...arflare.com,
	grundler@...isc-linux.org, arnd@...db.de, benh@...nel.crashing.org,
	avi@...hat.com, mtosatti@...hat.com,
	linux-net-drivers@...arflare.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH 2/5] drivers/net/ethernet/dec/tulip: Use standard
 __set_bit_le() function

2012/6/14 Takuya Yoshikawa <takuya.yoshikawa@...il.com>:
> On Wed, 13 Jun 2012 08:21:18 -0700
> Grant Grundler <grantgrundler@...il.com> wrote:
>
>> >> >> Should this hash_table be converted from u16 hash_table[32] to
>> >> >> DECLARE_BITMAP(hash_table, 16 * 32) to ensure that it is aligned
>> >> >> on long-word boundary?
>> >> >
>> >> > I think hash_table is already long-word aligned because it is placed
>> >> > right after a pointer.
>> >>
>> >> I recommend converting to proper bitmap.  Because such an implicit
>> >> assumption is easily broken by someone touching this function.
>> >
>> > Do you mean something like:
>> >        DECLARE_BITMAP(__hash_table, 16 * 32);
>> >        u16 *hash_table = (u16 *)__hash_table;
>> > ?
>> >
>> > Grant, what do you think about this?
>>
>> Hi Takuya,
>> two thoughts:
>> 1) while I agree with Akinobu and thank him for pointing out a
>> _potential_ alignment problem, this is a separate issue and your
>> existing patch should go in anyway. There are probably other drivers
>> with _potential_ alignment issues. Akinobu could get credit for
>> finding them by submitting patches after reviewing calls to set_bit
>> and set_bit_le() - similar to what you are doing now.
>
> I prefer approach 1.
>
> hash_table is local in build_setup_frame_hash(), so if further
> improvement is also required, we can do that locally there later.

This potential alignment problem is introduced by this patch.  Because
the original set_bit_le() in tulip driver can handle unaligned bitmap.
This is why I recommended it should be fixed in this patch.

But please just ignore me if I'm too much paranoid.  And I'll handle
this issue if no one wants to do it.
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