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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Mon, 8 Feb 2021 23:40:07 +0800
Message-ID: <CAMZfGtX-bHXoF_4rU+WzDNp+LmZj3CHWmurEvjCZBCyM2uiDMw@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v2] printk: fix deadlock when kernel panic
To:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc:     Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        john.ogness@linutronix.de,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Feb 8, 2021 at 9:12 PM Sergey Senozhatsky
<sergey.senozhatsky@gmail.com> wrote:
>
> On (21/02/08 16:49), Muchun Song wrote:
> > On Mon, Feb 8, 2021 at 2:38 PM Sergey Senozhatsky
> > <sergey.senozhatsky@gmail.com> wrote:
> > >
> > > 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?
> >
> > I took a closer look at the code. IIUC, It seems that there is no point
> > which decreases num_online_cpus.
>
> So then this never re-inits the safe_read_lock?

Right. If we encounter this case, we do not flush printk
buffer. So, it seems my previous patch is the right fix.
Right?

https://lore.kernel.org/patchwork/patch/1373563/

>
>                if (num_online_cpus() > 1)
>                        return;
>
>                debug_locks_off();
>                raw_spin_lock_init(&safe_read_lock);
>
>         -ss