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

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Date:   Tue, 19 Oct 2021 17:23:50 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, tj@...nel.org,
        gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
        minchan@...nel.org, jeyu@...nel.org, shuah@...nel.org,
        bvanassche@....org, dan.j.williams@...el.com, joe@...ches.com,
        tglx@...utronix.de, keescook@...omium.org, rostedt@...dmis.org,
        linux-spdx@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        ming.lei@...hat.com
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

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