Re: Removal of printk safe buffers delays NMI context printk

[Petr Mladek <pmladek@suse.com>]

On Fri 2021-11-05 11:26:35, Nicholas Piggin wrote:
>     printk: restore flushing of NMI buffers on remote CPUs after NMI backtraces
>     
>     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_flush() that can be called from non-NMI context on
>     another CPU to try to get those messages to the console.
>     
>     Fixes: 93d102f094be ("printk: remove safe buffers")
>     Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2206,6 +2206,26 @@ int vprintk_store(int facility, int level,
>  	return ret;
>  }
>  
> +void printk_flush(void)
> +{
> +	/*
> +	 * Disable preemption to avoid being preempted while holding
> +	 * console_sem which would prevent anyone from printing to
> +	 * console
> +	 */
> +	preempt_disable();
> +	/*
> +	 * Try to acquire and then immediately release the console
> +	 * semaphore.  The release will print out buffers and wake up
> +	 * /dev/kmsg and syslog() users.
> +	 */
> +	if (console_trylock_spinning())
> +		console_unlock();
> +	preempt_enable();
> +
> +	wake_up_klogd();
> +}
> +
>  asmlinkage int vprintk_emit(int facility, int level,
>  			    const struct dev_printk_info *dev_info,
>  			    const char *fmt, va_list args)
> --- 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_flush();

IMHO, nmi_trigger_cpumask_backtrace() might be called also from NMI
context.

printk_flush() is not safe in NMI context because console drivers have
their own locks. Even conosle_trylock() takes a lock, see
raw_spin_lock_irqsave(&sem->lock, flags) in down_trylock().

Best Regards,
Petr

> +
>  	clear_bit_unlock(0, &backtrace_flag);
>  	put_cpu();
>  }