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

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

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