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

[Max Krasnyanskiy]

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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