netdev - Re: [PATCH] bonding: clear header_ops when last slave detached (v2)

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Message-ID: <CAHA+R7PKRbBNwfUWRAsVHQreKsGXBc8CHorZRuyCfvwL4O=y+g@mail.gmail.com>
Date:	Thu, 20 Nov 2014 09:34:26 -0800
From:	Cong Wang <cwang@...pensource.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Wengang <wen.gang.wang@...cle.com>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] bonding: clear header_ops when last slave detached (v2)

On Wed, Nov 19, 2014 at 10:41 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2014-11-19 at 14:26 -0800, Cong Wang wrote:
>> On Tue, Nov 18, 2014 at 11:00 PM, Wengang <wen.gang.wang@...cle.com> wrote:
>> >
>> > Yes, that's true. So the simplest way is move ipoib_header_ops to vmlinux.
>> >
>>
>> That is not an option. Perhaps you need RCU to protect the dev->header_ops
>> pointer.
>
> This _is_ a reasonable option, especially for stable kernels
>
> ipoib_hard_header() is 100 bytes or less. Adding infrastructure all over
> the kernel to be able to use RCU or module refcounting will cost much
> more.

I didn't look into ipoib_header_ops, thought it might have some dependency
on symbols. So _generally_ speaking, we can't do that, the dependency
chain could be very long. It could be an exception and good luck
to play with module symbols.

>
> Tell me why it is ok for eth_header_ops() being static (while its _much_
> bigger), and not for ipoib_header_ops. This looks pretty arbitrary to
> me.
>

Simple, eth_* is kinda of a builtin library now, too many drivers use them.
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