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Date:   Mon, 8 Feb 2021 22:12:20 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        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>
Subject: Re: [External] Re: [PATCH v2] printk: fix deadlock when kernel panic
Message-ID: <YCE4tIrz/u/RkDc/@jagdpanzerIV.localdomain>
References: <20210206054124.6743-1-songmuchun@bytedance.com>
 <YCDcAy39BbPItdGY@jagdpanzerIV.localdomain>
 <CAMZfGtVBVSuH=HGNs7KFOtixSviy_stoZsiG4au0RUkUnH-0rQ@mail.gmail.com>
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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?

               if (num_online_cpus() > 1)
                       return;

               debug_locks_off();
               raw_spin_lock_init(&safe_read_lock);

	-ss