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Subject: Re: [RFC PATCH] panic: fix deadlock in panic()
To:     Petr Mladek <pmladek@suse.com>
CC:     <linux-kernel@vger.kernel.org>, <chenwandun@huawei.com>,
        <xiexiuqi@huawei.com>, <bobo.shaobowang@huawei.com>,
        <huawei.libin@huawei.com>, <sergey.senozhatsky@gmail.com>,
        <rostedt@goodmis.org>, "chengjian (D)" <cj.chengjian@huawei.com>
References: <20200603141915.38739-1-cj.chengjian@huawei.com>
 <20200604082947.GB22497@linux-b0ei>
From:   "chengjian (D)" <cj.chengjian@huawei.com>
Message-ID: <7b6f8522-f9b2-29e8-2dde-3bbfac19335b@huawei.com>
Date:   Fri, 5 Jun 2020 18:42:57 +0800
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Hi, Petr

On 2020/6/4 16:29, Petr Mladek wrote:

> It might cause double unlock (deadlock) on architectures that did not
> use NMI to stop the CPUs.
>
> I have created a conservative fix for this problem for SLES, see
> https://github.com/openSUSE/kernel-source/blob/SLE15-SP2-UPDATE/patches.suse/printk-panic-Avoid-deadlock-in-printk-after-stopping-CPUs-by-NMI.patch
> It solves the problem only on x86 architecture.
>
> There are many hacks that try to solve various scenarios but it
> is getting too complicated and does not solve all problems.

I have read your conservative fix and I have some question,

1-- does the console_sem need to be reinitialized ?

2-- Other architectures without NMI, is there no such problem ?

> The only real solution is lockless printk(). First piece is a lockless
> ringbuffer. See the last version at
> https://lore.kernel.org/r/20200501094010.17694-1-john.ogness@linutronix.de
>
> We prefer to work on the lockless solution instead of adding more
> complicated workarounds. This is why I even did not try to upstream
> the patch for SLES.
>
> In the meantime, you might also consider removing the offending
> message from the panic notifier if it is not really important.
>
> Best Regards,
> Petr
>
> .

Thank you.

     Cheng Jian