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Date:   Fri, 1 Oct 2021 14:23:48 +0200
From:   Petr Mladek <pmladek@suse.com>
To:     Alexander Popov <alex.popov@linux.com>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Joerg Roedel <jroedel@suse.de>,
        Maciej Rozycki <macro@orcam.me.uk>,
        Muchun Song <songmuchun@bytedance.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Kees Cook <keescook@chromium.org>,
        Luis Chamberlain <mcgrof@kernel.org>, Wei Liu <wl@xen.org>,
        John Ogness <john.ogness@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
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        Mark Rutland <mark.rutland@arm.com>,
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        Steven Rostedt <rostedt@goodmis.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
Message-ID: <20211001122348.GB965@pathway.suse.cz>
References: <20210929185823.499268-1-alex.popov@linux.com>
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 <20210929194924.GA880162@paulmck-ThinkPad-P17-Gen-1>
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 <a09c1d4d-1d5b-9092-ae3a-61bc22689dd2@linux.com>
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On Thu 2021-09-30 18:05:54, Alexander Popov wrote:
> On 30.09.2021 12:15, Petr Mladek wrote:
> > On Wed 2021-09-29 12:49:24, Paul E. McKenney wrote:
> >> On Wed, Sep 29, 2021 at 10:01:33PM +0300, Alexander Popov wrote:
> >>> This patch was tested using CONFIG_LKDTM.
> >>> The kernel kills a process that performs this:
> >>>   echo WARNING > /sys/kernel/debug/provoke-crash/DIRECT
> >>>
> >>> If you are fine with this approach, I will prepare a patch adding the
> >>> pkill_on_warn sysctl.
> >>
> >> I suspect that you need a list of kthreads for which you are better
> >> off just invoking panic().  RCU's various kthreads, for but one set
> >> of examples.
> > 
> > I wonder if kernel could survive killing of any kthread. I have never
> > seen a code that would check whether a kthread was killed and
> > restart it.
> 
> The do_group_exit() function calls do_exit() from kernel/exit.c, which is also
> called during a kernel oops. This function cares about a lot of special cases
> depending on the current task_struct. Is it fine?

IMHO, the bigger problem is that nobody will start the kthreads again.
As a result, some kernel functionality will not longer work.

User space threads are different. The user/admin typically
have a chance to start them again.

We might get inspiration in OOM killer. It never kills kthreads
and the init process, see oom_unkillable_task().

It would be better to panic() when WARN() is called from a kthread
or the init process.

Best Regards,
Petr