Re: [PATCH] serial: sh-sci: Use spin_{try}lock_irqsave instead of open coding version

[Daniel Wagner]

On 05/07/2018 02:47 PM, Sebastian Andrzej Siewior wrote:
> On 2018-05-03 09:43:33 [+0200], Geert Uytterhoeven wrote:
> > > --- a/drivers/tty/serial/sh-sci.c
> > > +++ b/drivers/tty/serial/sh-sci.c
> > > @@ -2516,13 +2516,12 @@ static void serial_console_write(struct console *co, const char *s,
> > >          unsigned long flags;
> > >          int locked = 1;
> > >
> > > -       local_irq_save(flags);
> >
> > Hence the below now runs with local interrupts enabled.
> >
> > For checking port->sysrq or oops_in_progress that probably isn't an issue.
> > If oops_in_progress is set, you have other problems, and the race condition
> > between checking the flag and calling spin_lock{,_irqsave}() existed before,
> > and is hard to avoid.
>
> while oops_in_progress is an issue of its own, the port->sysrq isn't
> avoided by by local_irq_save(). On SMP systems you can still receive a
> `break' signal on the UART and have a `printk()' issued on another CPU.
>
> > For actual console printing, I think you want to keep interrupts disabled.
>
> why? They should be disabled as part of getting the lock and not for any
> other reason.
>
> > >          if (port->sysrq)
> > >                  locked = 0;
> > >          else if (oops_in_progress)
> > > -               locked = spin_trylock(&port->lock);
> > > +               locked = spin_trylock_irqsave(&port->lock, flags);
> > >          else
> > > -               spin_lock(&port->lock);
> > > +               spin_lock_irqsave(&port->lock, flags);
> >
> > Add
> >
> >          if (!locked
> >                  local_irq_save(flags)
> >
> > here?
>
> So for oops_in_progress you get here with interrupts disabled. And if
> not, I don't see the point in disabling the interrupts without any kind
> of locking.

So I understand, the initial version of this patch was correct.

@Geert if you don't object I'll send a v3 (v1 ported to mainline).

Thanks,
Daniel