On Tue, Nov 02, 2021 at 04:56:10PM +0100, Petr Mladek wrote:
> On Tue 2021-11-02 22:59:30, Ming Lei wrote:
> > The completion finish is just for waiting release of the klp_patch
> > object, then releases module refcnt. We can simply drop the module
> > refcnt in the kobject release handler of klp_patch.
> >
> > This way also helps to support allocating klp_patch from heap.
>
> IMHO, this is wrong assumption. kobject_put() might do everyting
> asynchronously, see:
>
> kobject_put()
> kobject_release()
> INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);
> schedule_delayed_work(&kobj->release, delay);
>
> asynchronously:
>
> kobject_delayed_cleanup()
> kobject_cleanup()
> __kobject_del()
OK, this is one generic kobject release vs. module unloading issue to
solve, not unique for klp module, and there should be lots of drivers
suffering from it.
>
>
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > include/linux/livepatch.h | 1 -
> > kernel/livepatch/core.c | 12 +++---------
> > 2 files changed, 3 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > index 2614247a9781..9712818997c5 100644
> > --- a/include/linux/livepatch.h
> > +++ b/include/linux/livepatch.h
> > @@ -170,7 +170,6 @@ struct klp_patch {
> > bool enabled;
> > bool forced;
> > struct work_struct free_work;
> > - struct completion finish;
> > };
> >
> > #define klp_for_each_object_static(patch, obj) \
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index 335d988bd811..b967b4b0071b 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -551,10 +551,10 @@ static int klp_add_nops(struct klp_patch *patch)
> >
> > static void klp_kobj_release_patch(struct kobject *kobj)
> > {
> > - struct klp_patch *patch;
> > + struct klp_patch *patch = container_of(kobj, struct klp_patch, kobj);
> >
> > - patch = container_of(kobj, struct klp_patch, kobj);
> > - complete(&patch->finish);
> > + if (!patch->forced)
> > + module_put(patch->mod);
> > }
> >
> > static struct kobj_type klp_ktype_patch = {
> > @@ -678,11 +678,6 @@ static void klp_free_patch_finish(struct klp_patch *patch)
> > * cannot get enabled again.
> > */
> > kobject_put(&patch->kobj);
> > - wait_for_completion(&patch->finish);
> > -
> > - /* Put the module after the last access to struct klp_patch. */
> > - if (!patch->forced)
> > - module_put(patch->mod);
>
> klp_free_patch_finish() does not longer wait until the release
> callbacks are called.
>
> klp_free_patch_finish() is called also in klp_enable_patch() error
> path.
>
> klp_enable_patch() is called in module_init(). For example, see
> samples/livepatch/livepatch-sample.c
>
> The module must not get removed until the release callbacks are called.
> Does the module loader check the module reference counter when
> module_init() fails?
Good catch, that is really one corner case, in which the kobject has to
be cleaned up before returning from mod->init(), cause there can't be
module unloading in case of mod->init() failure.
Yeah, it should be more related with async kobject_put().
Also looks it is reasonable to add check when cleaning module loading
failure.
thanks,
Ming
