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SubjectRe: [PATCH] module: Fix race condition between load and unload module

On Sat, Apr 13, 2013 at 8:35 PM, Al Viro <> wrote:
> 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...

But kobject_release/kobject_cleanup function is called as a result of
atomic decrement_compare. Until we perform the atomic operation we
don't know whether it is final kobject_put() or not.

kobject_put() {
if (atomic_sub_and_test(kobj->kref->refcount)) {
// refcounter is decremented to 0 so cleanup sysfs

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