From: Muchun Song <songmuchun@bytedance.com>
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>
Subject: Re: [External] Re: [PATCH v2] printk: fix deadlock when kernel panic
Date: Mon, 8 Feb 2021 23:40:07 +0800 [thread overview]
Message-ID: <CAMZfGtX-bHXoF_4rU+WzDNp+LmZj3CHWmurEvjCZBCyM2uiDMw@mail.gmail.com> (raw)
In-Reply-To: <YCE4tIrz/u/RkDc/@jagdpanzerIV.localdomain>
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
next prev parent reply other threads:[~2021-02-08 18:13 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-06 5:41 [PATCH v2] printk: fix deadlock when kernel panic Muchun Song
2021-02-08 6:38 ` Sergey Senozhatsky
2021-02-08 8:49 ` [External] " Muchun Song
2021-02-08 13:12 ` Sergey Senozhatsky
2021-02-08 15:40 ` Muchun Song [this message]
2021-02-09 8:39 ` Petr Mladek
2021-02-10 2:25 ` Sergey Senozhatsky
2021-02-09 9:19 ` Petr Mladek
2021-02-09 12:20 ` [External] " Muchun Song
2021-02-10 2:16 ` Sergey Senozhatsky
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