Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

[Ming Lei <ming.lei@redhat.com>]

On Wed, Oct 20, 2021 at 10:19:27AM +0200, Miroslav Benes wrote:
> On Wed, 20 Oct 2021, Ming Lei wrote:
> 
> > On Wed, Oct 20, 2021 at 08:43:37AM +0200, Miroslav Benes wrote:
> > > On Tue, 19 Oct 2021, Ming Lei wrote:
> > > 
> > > > On Tue, Oct 19, 2021 at 08:23:51AM +0200, Miroslav Benes wrote:
> > > > > > > By you only addressing the deadlock as a requirement on approach a) you are
> > > > > > > forgetting that there *may* already be present drivers which *do* implement
> > > > > > > such patterns in the kernel. I worked on addressing the deadlock because
> > > > > > > I was informed livepatching *did* have that issue as well and so very
> > > > > > > likely a generic solution to the deadlock could be beneficial to other
> > > > > > > random drivers.
> > > > > > 
> > > > > > In-tree zram doesn't have such deadlock, if livepatching has such AA deadlock,
> > > > > > just fixed it, and seems it has been fixed by 3ec24776bfd0.
> > > > > 
> > > > > I would not call it a fix. It is a kind of ugly workaround because the 
> > > > > generic infrastructure lacked (lacks) the proper support in my opinion. 
> > > > > Luis is trying to fix that.
> > > > 
> > > > What is the proper support of the generic infrastructure? I am not
> > > > familiar with livepatching's model(especially with module unload), you mean
> > > > livepatching have to do the following way from sysfs:
> > > > 
> > > > 1) during module exit:
> > > > 	
> > > > 	mutex_lock(lp_lock);
> > > > 	kobject_put(lp_kobj);
> > > > 	mutex_unlock(lp_lock);
> > > > 	
> > > > 2) show()/store() method of attributes of lp_kobj
> > > > 	
> > > > 	mutex_lock(lp_lock)
> > > > 	...
> > > > 	mutex_unlock(lp_lock)
> > > 
> > > Yes, this was exactly the case. We then reworked it a lot (see 
> > > 958ef1e39d24 ("livepatch: Simplify API by removing registration step"), so 
> > > now the call sequence is different. kobject_put() is basically offloaded 
> > > to a workqueue scheduled right from the store() method. Meaning that 
> > > Luis's work would probably not help us currently, but on the other hand 
> > > the issues with AA deadlock were one of the main drivers of the redesign 
> > > (if I remember correctly). There were other reasons too as the changelog 
> > > of the commit describes.
> > > 
> > > So, from my perspective, if there was a way to easily synchronize between 
> > > a data cleanup from module_exit callback and sysfs/kernfs operations, it 
> > > could spare people many headaches.
> > 
> > kobject_del() is supposed to do so, but you can't hold a shared lock
> > which is required in show()/store() method. Once kobject_del() returns,
> > no pending show()/store() any more.
> > 
> > The question is that why one shared lock is required for livepatching to
> > delete the kobject. What are you protecting when you delete one kobject?
> 
> I think it boils down to the fact that we embed kobject statically to 
> structures which livepatch uses to maintain data. That is discouraged 
> generally, but all the attempts to implement it correctly were utter 
> failures.

OK, then it isn't one common usage, in which kobject covers the release
of the external object. What is the exact kobject in livepatching?

But kobject_del() won't release the kobject, you shouldn't need the lock
to delete kobject first. After the kobject is deleted, no any show() and
store() any more, isn't such sync[1] you expected?


Thanks,
Ming