linux-kernel - Re: [PATCH v2] printk: fix deadlock when kernel panic

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Message-ID: <YCNB492X/HcFT5xl@jagdpanzerIV.localdomain>
Date:   Wed, 10 Feb 2021 11:16:03 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        john.ogness@...utronix.de, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] printk: fix deadlock when kernel panic

On (21/02/09 10:19), Petr Mladek wrote:
> On Sat 2021-02-06 13:41:24, Muchun Song wrote:

[..]

> What about the following commit message? It uses imperative language
> and explains that the patch just prevents the deadlock. It removes
> some details. The diagram is better than many words.
> 
> <commit message>
> printk_safe_flush_on_panic() caused the following deadlock on our server:
> 
> CPU0:                                         CPU1:
> panic                                         rcu_dump_cpu_stacks
>   kdump_nmi_shootdown_cpus                      nmi_trigger_cpumask_backtrace
>     register_nmi_handler(crash_nmi_callback)      printk_safe_flush
>                                                     __printk_safe_flush
>                                                       raw_spin_lock_irqsave(&read_lock)
>     // send NMI to other processors
>     apic_send_IPI_allbutself(NMI_VECTOR)
>                                                         // NMI interrupt, dead loop
>                                                         crash_nmi_callback
>   printk_safe_flush_on_panic
>     printk_safe_flush
>       __printk_safe_flush
>         // deadlock
>         raw_spin_lock_irqsave(&read_lock)

[..]

I would also add to the commit message that it avoids the deadlock
_in this particular case_ at expense of losing contents of printk_safe
buffers. This looks important enough to be mentioned.

	-ss