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

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Date:   Tue, 2 Nov 2021 22:56:14 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        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,
        live-patching@...r.kernel.org, ming.lei@...hat.com
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

On Tue, Nov 02, 2021 at 03:15:15PM +0100, Petr Mladek wrote:
> 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().

After kobject_del() returns, no one can call run into show()/store(),
and all pending show()/store() are drained meantime. But yes, the release
handler may still be called later, and the kobject has to be freed
during or before module_exit().

https://lore.kernel.org/lkml/20211101112548.3364086-2-ming.lei@redhat.com/

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

No, no one can do that after kobject_del() returns.

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

But so far klp_patch can only be defined as static instance, and it
depends on the implementation, especially the release handler.


Thanks,
Ming