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SubjectRe: 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.


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