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

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