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SubjectRe: [PATCH v2] printk: fix deadlock when kernel panic
On (21/02/09 10:19), Petr Mladek wrote:
> On Sat 2021-02-06 13:41:24, Muchun Song wrote:

[..]

> What about the following commit message? It uses imperative language
> and explains that the patch just prevents the deadlock. It removes
> some details. The diagram is better than many words.
>
> <commit message>
> printk_safe_flush_on_panic() caused the following deadlock on our server:
>
> 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
> printk_safe_flush_on_panic
> printk_safe_flush
> __printk_safe_flush
> // deadlock
> raw_spin_lock_irqsave(&read_lock)

[..]

I would also add to the commit message that it avoids the deadlock
_in this particular case_ at expense of losing contents of printk_safe
buffers. This looks important enough to be mentioned.

-ss

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