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

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Date: Tue, 21 May 2024 14:37:08 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v5 26/30] printk: nbcon: Implement emergency
 sections

On Mon 2024-05-06 12:45:59, John Ogness wrote:
> On 2024-05-02, John Ogness <john.ogness@...utronix.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:

Great catch!

> 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)--;

Looks good.

> 	preempt_enable();
> 
> 	if (do_trigger_flush)
> 		printk_trigger_flush();
> }

Best Regards,
Petr