Re: [PATCH v3] printk: fix deadlock when kernel panic

[Petr Mladek]

On Wed 2021-02-10 11:48:23, Muchun Song wrote:
> 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)
> 
> DEADLOCK: read_lock is taken on CPU1 and will never get released.
> 
> It happens when panic() stops a CPU by NMI while it has been in
> the middle of printk_safe_flush().
> 
> Handle the lock the same way as logbuf_lock. The printk_safe buffers
> are flushed only when both locks can be safely taken. It can avoid
> the deadlock _in this particular case_ at expense of losing contents
> of printk_safe buffers.
> 
> Note: It would actually be safe to re-init the locks when all CPUs were
>       stopped by NMI. But it would require passing this information
>       from arch-specific code. It is not worth the complexity.
>       Especially because logbuf_lock and printk_safe buffers have been
>       obsoleted by the lockless ring buffer.
> 
> Fixes: cf9b1106c81c ("printk/nmi: flush NMI messages on the system panic")
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

The patch is committed on printk/linux.git, branch for-5.12.

Best Regards,
Petr