linux-kernel - Re: Module loading/unloading and "The Stop Machine"

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Date:	Thu, 21 Feb 2008 18:16:40 -0800
From:	Max Krasnyanskiy <maxk@...lcomm.com>
To:	Tejun Heo <htejun@...il.com>
CC:	rusty@...tcorp.com.au, Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Module loading/unloading and "The Stop Machine"

Tejun Heo wrote:
> Max Krasnyanskiy wrote:
>> Tejun Heo wrote:
>>> Max Krasnyanskiy wrote:
>>>> Thanks for the info. I guess I missed that from the code. In any case
>>>> that seems like a pretty heavy refcounting mechanism. In a sense that
>>>> every time something is loaded or unloaded entire machine freezes,
>>>> potentially for several milliseconds. Normally it's not a big deal. But
>>>> once you get more and more CPUs and/or start using realtime apps this
>>>> becomes a big deal.
>>> Module loading doesn't involve stop_machine last time I checked.  It's a
>>> big deal when unloading a module but it's actually a very good trade off
>>> because it makes much hotter path (module_get/put) much cheaper.  If
>>> your application can't stand stop_machine, simply don't unload a module.
>> static struct module *load_module(void __user *umod,
>>                                  unsigned long len,
>>                                  const char __user *uargs)
>> {
>>      ...
>>
>>      /* Now sew it into the lists so we can get lockdep and oops
>>         * info during argument parsing.  Noone should access us, since
>>         * strong_try_module_get() will fail. */
>>        stop_machine_run(__link_module, mod, NR_CPUS);
>>      ...
>> }
> 
> Ah... right.  That part doesn't have anything to do with module
> reference counting as the comment suggests and can probably be removed
> by updating how kallsyms synchronize against module load/unload.

That list (updated by __link_module) is accessed in couple of other places. ie outside symbol
lookup stuff used for kallsyms.

Max
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