linux-kernel - Re: [PULL] modules-next

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Date:	Mon, 22 Dec 2014 11:51:10 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ionut Alexa <ionut.m.alexa@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PULL] modules-next

Linus Torvalds <torvalds@...ux-foundation.org> writes:
> On Thu, Dec 18, 2014 at 4:55 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>>
>> The exciting thing here is the getting rid of stop_machine on module
>> removal.  This is possible by using a simple atomic_t for the counter,
>> rather than our fancy per-cpu counter: it turns out that no one is doing
>> a module increment per net packet, so the slowdown should be in the noise.
>
> Famous last words. It may not happen per-packet, but I see
> module_get() in various block drivers and in netfilter code etc, and
> some of them may be pretty bad.
>
> Let's see how it all works out.

Indeed.  The general pattern is "get on open/init"; get-on-every-use was
most useful combined with blocking rmmod, which we removed a few kernels
ago (because no one used it).

I did a random audit until I got bored, and I put a printk-on-module-get
in my kernels for a while, and none of my usecases flood my logs.  But
it's definitely a risk...

Cheers,
Rusty.
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