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

Ming Lei
2021-10-19T09:24:26Z

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)

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);

Or can you explain your requirement on kobject/module unload in a bit
details?

Thanks,
Ming