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

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

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?

>  
> > IMO, the above usage simply caused AA deadlock. Even in Luis's patch
> > 'zram: fix crashes with cpu hotplug multistate', new/same AA deadlock
> > (hot_remove_store() vs. disksize_store() or reset_store()) is added
> > because hot_remove_store() isn't called from module_exit().
> > 
> > Luis tries to delay unloading module until all show()/store() are done. But
> > that can be obtained by the following way simply during module_exit():
> > 
> > 	kobject_del(lp_kobj); //all pending store()/show() from lp_kobj are done,
> > 						  //no new store()/show() can come after
> > 						  //kobject_del() returns	
> > 	mutex_lock(lp_lock);
> > 	kobject_put(lp_kobj);
> > 	mutex_unlock(lp_lock);
> 
> kobject_del() already calls kobject_put(). Did you mean __kobject_del(). 
> That one is internal though.

kobject_del() is counter-part of kobject_add(), and kobject_put() will
call kobject_del() automatically() if it isn't deleted yet, but usually
kobject_put() is for releasing the object only. It is more often to
release kobject by calling kobject_del() and kobject_put().

>  
> > Or can you explain your requirement on kobject/module unload in a bit
> > details?
> 
> Does the above makes sense?

I think now focus is the shared lock between kobject_del() and
show()/store() of the kobject's attributes.


Thanks,
Ming