Re: Module Refcount & Stuff mini-FAQ

[Werner Almesberger]

Rusty Russell wrote:
> Yes, but between doing and undoing (in the failure path) someone has
> started using the module.

But how ? Don't we have only two ways of calling a module, i.e.
by symbol, or by callback ? All callbacks that might call a module
must be protected with try_module_get, right ? (*)

(*) Actually, if the registration can be revoked, and the
    deregistration function does properly synchronize with on-going
    callbacks, you shouldn't need try_module_get either. E.g.
    del_timer_sync doesn't need to know about module owners.

So, if you make try_module_get work during initialization, and
modules don't publish their symbols before initialization is done,
there should be no problem ?

> Given we have a method of isolating a module already, it seems logical
> to use it to prevent exactly this race.

But that's mainly the symbol-unload race, considering that any call
through a function pointer requires try_module_get anyway. Without
try_module_get, there would also be callback-unload races if
callback de-registration doesn't synchronize (e.g. del_timer
vs. del_timer_sync).

By the way, it's also not so nice that there can't be
callbacks at removal, e.g.

service_unregister(...)
{
	...
	for_pending_requests(req) {
		...
		if (try_module_get(req->owner))
			req->fn(req,REQUEST_CANCELLED);
		else
			printk(KERN_CRIT "we just dropped a request on the "
			  "floor, how nice\n");
		...
	}
	...
}

Calling this from the module removal function would be
perfectly safe.

Actually ... can't you allow modules to be called until the
cleanup function has returned ?

> Unfortunately my last attempt
> assumed noone did this, and broke IDE and SCSI (hence pissing
> *everyone* off 8).

Two in one strike ain't bad ;-) Maybe we can find something that
gets rid of that pesky NFS, too, e.g. by adding

#define while if    /* enforce efficient programming practices */

to linux/kernel.h, or such ;-)

- Werner

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 / Werner Almesberger, Buenos Aires, Argentina         [e-mail] /
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