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Date:   Tue, 26 Oct 2021 10:01:40 -0700
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     Ming Lei <ming.lei@redhat.com>,
        Julia Lawall <julia.lawall@inria.fr>
Cc:     Petr Mladek <pmladek@suse.com>, Miroslav Benes <mbenes@suse.cz>,
        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
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Sender: Luis Chamberlain <mcgrof@infradead.org>
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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.

Never?

> > 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.

I'm working on a generic coccinelle patch which hunts for actual cases
using iteration (a feature of coccinelle for complex searches). The
search is pretty involved, so I don't think I'll have an answer to this
soon.

Since the question of how generic this deadlock is remains questionable,
I think it makes sense to put the generic deadlock fix off the table for
now, and we address this once we have a more concrete search with
coccinelle.

But to say we *don't* have drivers which can cause this is obviously
wrong as well, from a cursory search so far. But let's wait and see how
big this list actually is.

I'll drop the deadlock generic fixes and move on with at least a starter
kernfs / sysfs tests.

  Luis