linux-kernel - Re: [PATCH V4 1/3] livepatch: remove 'struct completion finish' from klp

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Date:   Wed, 3 Nov 2021 08:51:32 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>, ming.lei@...hat.com
Subject: Re: [PATCH V4 1/3] livepatch: remove 'struct completion finish' from
 klp_patch

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@...hat.com>
> > ---
> >  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