Re: [PATCH] printk: restore flushing of NMI buffers on remote CPUs after NMI backtraces

[John Ogness <john.ogness@linutronix.de>]

On 2021-11-07, Nicholas Piggin <npiggin@gmail.com> wrote:
> printk from NMI context relies on irq work being raised on the local CPU
> to print to console. This can be a problem if the NMI was raised by a
> lockup detector to print lockup stack and regs, because the CPU may not
> enable irqs (because it is locked up).
>
> Introduce printk_trigger_flush() that can be called another CPU to try
> to get those messages to the console, call that where printk_safe_flush
> was previously called.
>
> Fixes: 93d102f094be ("printk: remove safe buffers")
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Reviewed-by: John Ogness <john.ogness@linutronix.de>

> ---
>  arch/powerpc/kernel/watchdog.c | 6 ++++++
>  include/linux/printk.h         | 4 ++++
>  kernel/printk/printk.c         | 5 +++++
>  lib/nmi_backtrace.c            | 6 ++++++
>  4 files changed, 21 insertions(+)
>
> diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
> index 5f69ba4de1f3..c8017bc23b00 100644
> --- a/arch/powerpc/kernel/watchdog.c
> +++ b/arch/powerpc/kernel/watchdog.c
> @@ -227,6 +227,12 @@ static void watchdog_smp_panic(int cpu)
>  		cpumask_clear(&wd_smp_cpus_ipi);
>  	}
>  
> +	/*
> +	 * Force flush any remote buffers that might be stuck in IRQ context
> +	 * and therefore could not run their irq_work.
> +	 */
> +	printk_trigger_flush();
> +
>  	if (hardlockup_panic)
>  		nmi_panic(NULL, "Hard LOCKUP");
>  
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 85b656f82d75..9497f6b98339 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -198,6 +198,7 @@ void dump_stack_print_info(const char *log_lvl);
>  void show_regs_print_info(const char *log_lvl);
>  extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
>  extern asmlinkage void dump_stack(void) __cold;
> +void printk_trigger_flush(void);
>  #else
>  static inline __printf(1, 0)
>  int vprintk(const char *s, va_list args)
> @@ -274,6 +275,9 @@ static inline void dump_stack_lvl(const char *log_lvl)
>  static inline void dump_stack(void)
>  {
>  }
> +static inline void printk_trigger_flush(void)
> +{
> +}
>  #endif
>  
>  #ifdef CONFIG_SMP
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index a8d0a58deebc..99221b016c68 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3252,6 +3252,11 @@ void defer_console_output(void)
>  	preempt_enable();
>  }
>  
> +void printk_trigger_flush(void)
> +{
> +	defer_console_output();
> +}
> +
>  int vprintk_deferred(const char *fmt, va_list args)
>  {
>  	int r;
> diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
> index f9e89001b52e..199ab201d501 100644
> --- a/lib/nmi_backtrace.c
> +++ b/lib/nmi_backtrace.c
> @@ -75,6 +75,12 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
>  		touch_softlockup_watchdog();
>  	}
>  
> +	/*
> +	 * Force flush any remote buffers that might be stuck in IRQ context
> +	 * and therefore could not run their irq_work.
> +	 */
> +	printk_trigger_flush();
> +
>  	clear_bit_unlock(0, &backtrace_flag);
>  	put_cpu();
>  }
> -- 
> 2.23.0