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

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

-ss

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