Hi Johan,

On Fri, Apr 16, 2021 at 4:35 PM Johan Hovold [off-list ref] wrote:

On Tue, Apr 13, 2021 at 07:44:39AM +0800, dillon min wrote:

quoted

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.

quoted

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,

quoted

else
      spin_lock_irqsave(&port->lock, flags);

....

if (locked)
     spin_unlock_irqrestore(&port->lock, flags);

Johan

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