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

From: Petr Mladek <pmladek@suse.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: Miroslav Benes <mbenes@suse.cz>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	tj@kernel.org, gregkh@linuxfoundation.org,
	akpm@linux-foundation.org, minchan@kernel.org, jeyu@kernel.org,
	shuah@kernel.org, bvanassche@acm.org, dan.j.williams@intel.com,
	joe@perches.com, tglx@linutronix.de, keescook@chromium.org,
	rostedt@goodmis.org, linux-spdx@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-kernel@vger.kernel.org, live-patching@vger.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate
Date: Tue, 2 Nov 2021 15:15:15 +0100	[thread overview]
Message-ID: <YYFH85CmVOYIMdYh@alley> (raw)
In-Reply-To: <YXgguuAY5iEUIV0u@T590>

On Tue 2021-10-26 23:37:30, Ming Lei wrote:
> On Tue, Oct 26, 2021 at 10:48:18AM +0200, Petr Mladek wrote:
> > Below are more details about the livepatch code. I hope that it will
> > help you to see if zram has similar problems or not.
> > 
> > We have kobject in three structures: klp_func, klp_object, and
> > klp_patch, see include/linux/livepatch.h.
> > 
> > These structures have to be statically defined in the module sources
> > because they define what is livepatched, see
> > samples/livepatch/livepatch-sample.c
> > 
> > The kobject is used there to show information about the patch, patched
> > objects, and patched functions, in sysfs. And most importantly,
> > the sysfs interface can be used to disable the livepatch.
> > 
> > The problem with static structures is that the module must stay
> > in the memory as long as the sysfs interface exists. It can be
> > solved in module_exit() callback. It could wait until the sysfs
> > interface is destroyed.
> > 
> > kobject API does not support this scenario. The relase() callbacks
> 
> kobject_delete() is for supporting this scenario, that is why we don't
> need to grab module refcnt before calling show()/store() of the
> kobject's attributes.
> 
> kobject_delete() can be called in module_exit(), then any show()/store()
> will be done after kobject_delete() returns.

I am a bit confused. I do not see kobject_delete() anywhere in kernel
sources.

I see only kobject_del() and kobject_put(). AFAIK, they do _not_
guarantee that either the sysfs interface was destroyed or
the release callbacks were called. For example, see
schedule_delayed_work(&kobj->release, delay) in kobject_release().

By other words, anyone could still be using either the sysfs interface
or the related structures after kobject_del() or kobject_put()
returns.

IMHO, kobject API does not support static structures and module
removal.

Best Regards,
Petr