[PATCH v2] serial: stm32: optimize spin lock usage

[dillon min]

Hi Johan,

On Fri, Apr 16, 2021 at 4:35 PM Johan Hovold <johan at kernel.org> wrote:
>
> On Tue, Apr 13, 2021 at 07:44:39AM +0800, dillon min wrote:
> > Hi Johan, Erwan
> >
> > It seems still a bit of a problem in the current version, not deadlock
> > but access register at the same time.
> >
> > For driver , we should consider it running under smp, let's think
> > about it for this case:
> >
> > static void stm32_usart_console_write(struct console *co, const char *s,
> >                                       unsigned int cnt)
> > {
> >          .....
> >          local_irq_save(flags);
> >          if (port->sysrq)
> >                     locked = 0;
> >          .....
> >          access register cr1, tdr, isr
> >          .....
> >
> >          local_irq_restore(flags);
> > }
> >
> > if port->sysrq is 1, stm32_usart_console_write() just disable local
> > irq response by local_irq_save(), at the time of access register cr1,
> > tdr, isr. an TXE interrupt raised, for other cores(I know stm32
> > mpu/mcu do not have multi cores, just assume it has), it still has a
> > chance to handle interrupt.  Then there is no lock to protect the uart
> > register.
>
> Right, the sysrq handling is a bit of a hack.
>
> > changes to below, should be more safe:
> >
> > .....
> > if (port->sysrq || oops_in_progress)
> >       locked = spin_trylock_irqsave(&port->lock, flags);
>
> Except that the lock debugging code would detect the attempt at
> recursive locking here and complain loudly on UP.
>
> If you really want to fix this, we have uart_unlock_and_check_sysrq()
> which can be used to defer sysrq processing until the interrupt handler
> has released the lock.

Great, uart_unlock_and_check_sysrq() is fit to fix this. you mean make
the flow like below:

    stm32_usart_threaded_interrupt()
      spin_lock(&port->lock);
      uart_unlock_and_check_sysrq(port, flags);
      ...
      uart_prepare_sysrq_char();
          printk();
            stm32_usart_console_write();
              locked = spin_trylock_irqsave(&port->lock); //only
handle oops, normal case

If so, I will submit v3 as you suggested. thanks.

Best regards.
Dillon,
>
> > else
> >       spin_lock_irqsave(&port->lock, flags);
> >
> > ....
> >
> > if (locked)
> >      spin_unlock_irqrestore(&port->lock, flags);
>
> Johan