Re: [PATCH printk v5 26/30] printk: nbcon: Implement emergency sections

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

On 2024-05-02, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 8f6b3858ab27..991e2702915c 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> +void nbcon_cpu_emergency_exit(void)
> +{
> +	unsigned int *cpu_emergency_nesting;
> +	bool do_trigger_flush = false;
> +
> +	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> +
> +	/*
> +	 * Flush the messages before enabling preemtion to see them ASAP.
> +	 *
> +	 * Reduce the risk of potential softlockup by using the
> +	 * flush_pending() variant which ignores messages added later. It is
> +	 * called before decrementing the counter so that the printing context
> +	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
> +	 */
> +	if (*cpu_emergency_nesting == 1) {
> +		nbcon_atomic_flush_pending();
> +		do_trigger_flush = true;
> +	}
> +
> +	(*cpu_emergency_nesting)--;
> +
> +	if (WARN_ON_ONCE(*cpu_emergency_nesting < 0))
> +		*cpu_emergency_nesting = 0;

I see two issues here. First, this is unsigned. kernel test robot
reported:

> kernel/printk/nbcon.c:1540 nbcon_cpu_emergency_exit() warn: unsigned
> '*cpu_emergency_nesting' is never less than zero.

Also, in this situation, we are allowing a brief window of activated
emergency mode by allowing it to become !0 before correcting
it. Instead, we should avoid illegally decrementing. I suggest:

void nbcon_cpu_emergency_exit(void)
{
	unsigned int *cpu_emergency_nesting;
	bool do_trigger_flush = false;

	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();

	/*
	 * Flush the messages before enabling preemtion to see them ASAP.
	 *
	 * Reduce the risk of potential softlockup by using the
	 * flush_pending() variant which ignores messages added later. It is
	 * called before decrementing the counter so that the printing context
	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
	 */
	if (*cpu_emergency_nesting == 1) {
		nbcon_atomic_flush_pending();
		do_trigger_flush = true;
	}

	if (!WARN_ON_ONCE(*cpu_emergency_nesting == 0))
		(*cpu_emergency_nesting)--;

	preempt_enable();

	if (do_trigger_flush)
		printk_trigger_flush();
}

John