Hi Johan, On Fri, Apr 16, 2021 at 4:35 PM Johan Hovold <johan@xxxxxxxxxx> 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