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From: Rusty Russell <rusty@rustcorp.com.au>
To: Andi Kleen <andi@firstfloor.org>
Subject: Re: Module loading/unloading and "The Stop Machine"
Date: Tue, 4 Mar 2008 12:21:05 +1100
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Cc: Max Krasnyanskiy <maxk@qualcomm.com>, Tejun Heo <htejun@gmail.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Kathy Staples <kathy.staples@au1.ibm.com>
References: <47ABC08C.8010101@qualcomm.com> <47BE29A0.6040409@qualcomm.com> <p73prup8awh.fsf@bingen.suse.de>
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On Friday 22 February 2008 22:53:50 Andi Kleen wrote:
> Max Krasnyanskiy <maxk@qualcomm.com> writes:
> > 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);
> >      ...
> > }
>
> Wow you found some really bad code. I bet it wouldn't be that
> difficult to fix the code to allow oops safe list insertion
> without using the big stop machine overkill hammer.

Yes it's overkill and it's becoming more noticable with larger machines.  
(Also, stop_machine is heavier than it should be, Kathy is working on fixing 
that though).

Most obvious is to add __ "lockless, but I'm in trouble anyway" accessors, but 
last I looked they'd have to be plumbed through a fair bit of code.

>>From a quick reading of the code, a lockless add is possible, but it's fragile 
if one of the readers does something more complex in future.

Thanks,
Rusty.