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SubjectRe: [PATCH] module: Fix race condition between load and unload module
On Fri, Apr 12, 2013 at 04:47:50PM -0700, Linus Torvalds wrote:
> This is a much more generic bug in kobjects, and I would hate to add
> some random workaround for just one case of this bug like you do. The
> more fundamental bug needs to be fixed too.
>
> I think the more fundamental bugfix is to just fix kobject_get() to
> return NULL if the refcount was zero, because in that case the kobject
> no longer really exists.
>
> So instead of having
>
> kref_get(&kobj->kref);
>
> it should do
>
> if (!atomic_inc_not_zero(&kobj->kref.refcount))
> kobj = NULL;
>
> and I think that should fix your race automatically, no? Proper patch
> attached (but TOTALLY UNTESTED - it seems to compile, though).
>
> The problem is that we lose the warning for when the refcount is zero
> and somebody does a kobject_get(), but that is ok *assuming* that
> people actually check the return value of kobject_get() rather than
> just "know" that if they passed in a non-NULL kobj, they'll get it
> right back.
>
> Greg - please take a look... I'm adding Al to the discussion too,
> because Al just *loooves* these kinds of races ;)

Unless I'm misreading what's going on, we have the following to thank for that:
/* remove from sysfs if the caller did not do it */
if (kobj->state_in_sysfs) {
pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
kobject_name(kobj), kobj);
kobject_del(kobj);
}
in kobject_cleanup(). Why don't we require kobject_del() before the final
kobject_put(), if the sucker had been added? FWIW, I thought it *was*
required all along...


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