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Date:   Mon, 8 Feb 2021 15:38:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     pmladek@suse.com, sergey.senozhatsky@gmail.com,
        rostedt@goodmis.org, john.ogness@linutronix.de,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] printk: fix deadlock when kernel panic
Message-ID: <YCDcAy39BbPItdGY@jagdpanzerIV.localdomain>
References: <20210206054124.6743-1-songmuchun@bytedance.com>
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In-Reply-To: <20210206054124.6743-1-songmuchun@bytedance.com>
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On (21/02/06 13:41), Muchun Song wrote:
> We found a deadlock bug on our server when the kernel panic. It can be
> described in the following diagram.
> 
> 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

At what point does this decrement num_online_cpus()? Any chance that
panic CPU can apic_send_IPI_allbutself() and printk_safe_flush_on_panic()
before num_online_cpus() becomes 1?

>   printk_safe_flush_on_panic
>     printk_safe_flush
>       __printk_safe_flush
>         // deadlock
>         raw_spin_lock_irqsave(&read_lock)

	-ss