Re: Linux 2.4.1-ac15

[Philipp Rumpf]

On Mon, 19 Feb 2001, Keith Owens wrote:
> On Mon, 19 Feb 2001 11:35:08 +0000 (GMT), 
> Alan Cox <[e-mail]> wrote:
> >The problem isnt running module code. What happens in this case
> >
> >        mod->next = module_list;
> >        module_list = mod;      /* link it in */
> >
> >Note no write barrier.
> 
> <humour>It works on ix86 so the code must be right</humour>

Too bad it doesn't.

> >Delete is even worse
> >
> >We unlink the module
> >We free the memory
> >
> >At the same time another cpu may be walking the exception table that we free.
> 
> Another good reason why locking modules via use counts from external
> code is not the right fix.  We definitely need a quiesce model for
> module removal.

Unless I'm mistaken, we need both use counts and SMP magic (though not
necessarily as extreme as what the "freeze all other CPUs during module
unload" patch did).

I think something like this would work (in addition to use counts)

int callin_func(void *p)
{
	int *cpu = p;

	while (*cpu != smp_processor_id()) {
		current->cpus_allowed = 1 << *cpu;
		schedule();
	}

	return 0;
}

void callin_other_cpus(void)
{
	int cpus[smp_num_cpus];
	int i;

	for (i=0; i<smp_num_cpus; i++) {
		cpus[i] = i;

		kernel_thread(callin_func, &cpus[i], ...);
	}
}

and call callin_other_cpus() before unloading a module.

I'm not sure how you could make exception handling safe without locking
all accesses to the module list - but that sounds like the sane thing to
do anyway.