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

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Date:   Tue, 2 Nov 2021 16:24:06 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Ming Lei <ming.lei@...hat.com>,
        Julia Lawall <julia.lawall@...ia.fr>,
        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,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

On Wed 2021-10-27 13:57:40, Miroslav Benes wrote:
> On Tue, 26 Oct 2021, Luis Chamberlain wrote:
> 
> > On Tue, Oct 26, 2021 at 11:37:30PM +0800, Ming Lei wrote:
> > > On Tue, Oct 26, 2021 at 10:48:18AM +0200, Petr Mladek wrote:
> > > > Livepatch code never called kobject_del() under a lock. It would cause
> > > > the obvious deadlock.

I have to correct myself. IMHO, the deadlock is far from obvious. I
always get lost in the code and the documentation is not clear.
I always get lost.

> >
> > Never?
> 
> kobject_put() to be precise.

IMHO, the problem is actually with kobject_del() that gets blocked
until the sysfs interface gets removed. kobject_put() will have
the same problem only when the clean up is not delayed.


> When I started working on the support for module/live patches removal, 
> calling kobject_put() under our klp_mutex lock was the obvious first 
> choice given how the code was structured, but I ran into problems with 
> deadlocks immediately. So it was changed to async approach with the 
> workqueue. Thus the mainline code has never suffered from this, but we 
> knew about the issues.
>  
> > > > The historic code only waited in the
> > > > module_exit() callback until the sysfs interface was removed.
> > > 
> > > OK, then Luis shouldn't consider livepatching as one such issue to solve
> > > with one generic solution.
> > 
> > It's not what I was told when the deadlock was found with zram, so I was
> > informed quite the contrary.
> 
> >From my perspective, it is quite easy to get it wrong due to either a lack 
> of generic support, or missing rules/documentation. So if this thread 
> leads to "do not share locks between a module removal and a sysfs 
> operation" strict rule, it would be at least something. In the same 
> manner as Luis proposed to document try_module_get() expectations.

The rule "do not share locks between a module removal and a sysfs
operation" is not clear to me.

IMHO, there are the following rules:

1. rule: kobject_del() or kobject_put() must not be called under a lock that
	 is used by store()/show() callbacks.

   reason: kobject_del() waits until the sysfs interface is destroyed.
	 It has to wait until all store()/show() callbacks are finished.


2. rule: kobject_del()/kobject_put() must not be called from the
	related store() callbacks.

   reason: same as in 1st rule.


3. rule: module_exit() must wait until all release() callbacks are called
	 when kobject are static.

   reason: kobject_put() must be called to clean up internal
	dependencies. The clean up might be done asynchronously
	and need access to the kobject structure.


Best Regards,
Petr

PS: I am sorry if I am messing things. I want to be sure that we are
    all talking about the same and understand it the same way.