Re: Module loading/unloading and "The Stop Machine"

[Rusty Russell <rusty@rustcorp.com.au>]

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.