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

[Muchun Song]

On Mon, Feb 8, 2021 at 2:38 PM Sergey Senozhatsky
<sergey.senozhatsky@xxxxxxxxx> 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.

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