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

Petr Mladek
2024-05-21T12:37:12Z

On Mon 2024-05-06 12:45:59, John Ogness wrote:
> 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:

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