Re: [patch RFC 28/29] printk: Provide functions for atomic write enforcement

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

Below is a fix that was used for the LPC2022 demo so that atomic
printing occurs in NMI context.

On 2022-09-11, Thomas Gleixner <tglx@linutronix.de> wrote:
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1072,6 +1072,8 @@ static inline void log_buf_add_cpu(void)
>  #endif /* CONFIG_SMP */
>  
>  static void cons_alloc_percpu_data(struct console *con);
> +static void cons_atomic_flush(void);
> +static void cons_wake_threads(void);
>  
>  static void __init set_percpu_data_ready(void)
>  {
> @@ -2270,17 +2272,21 @@ asmlinkage int vprintk_emit(int facility
>  
>  	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
>  
> +	/*
> +	 * The caller may be holding system-critical or
> +	 * timing-sensitive locks. Disable preemption during
> +	 * printing of all remaining records to all consoles so that
> +	 * this context can return as soon as possible. Hopefully
> +	 * another printk() caller will take over the printing.
> +	 */
> +	preempt_disable();
> +
> +	/* Flush the non-BKL consoles if required */
> +	cons_atomic_flush();
> +
>  	/* If called from the scheduler, we can not call up(). */
>  	if (!in_sched) {
>  		/*
> -		 * The caller may be holding system-critical or
> -		 * timing-sensitive locks. Disable preemption during
> -		 * printing of all remaining records to all consoles so that
> -		 * this context can return as soon as possible. Hopefully
> -		 * another printk() caller will take over the printing.
> -		 */
> -		preempt_disable();
> -		/*
>  		 * Try to acquire and then immediately release the console
>  		 * semaphore. The release will print out buffers. With the
>  		 * spinning variant, this context tries to take over the
> @@ -2288,9 +2294,11 @@ asmlinkage int vprintk_emit(int facility
>  		 */
>  		if (console_trylock_spinning())
>  			console_unlock();
> -		preempt_enable();
>  	}
>  
> +	preempt_enable();
> +
> +	cons_wake_threads();
>  	wake_up_klogd();
>  	return printed_len;
>  }

Atomic flushing also needs to occur for the cases where deferred
printing is used (NMI or explicit defer). The following hunk will take
care of that.

@@ -3648,6 +3658,8 @@ void wake_up_klogd(void)
 
 void defer_console_output(void)
 {
+	cons_atomic_flush();
+
 	/*
 	 * New messages may have been added directly to the ringbuffer
 	 * using vprintk_store(), so wake any waiters as well.

John Ogness