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

[Hua Dillon]

Hi Erwan,

Erwan LE RAY <erwan.leray at foss.st.com> 于2021年4月16日周五 上午1:10写道：
>
> Hi Dillon,
>
> STM32MP151 is mono-core, but both STM32MP153 and STM32MP157 are
> dual-core (see
> https://www.st.com/content/st_com/en/products/microcontrollers-microprocessors/stm32-arm-cortex-mpus.html).
> So your point is fully relevant, thanks.

Thanks.
>
> ST already fixed the same issue in st-asc.c driver in the past (see
> ef49ffd8), because a systematic deadlock was detected with RT kernel.
>
> You proposed a first implementation in your patch, and a second one in
> the discussion. It seems that your initial proposal (ie your V2 patch)
> is the most standard one (implemented in 6 drivers). The second
> implementation is implemented by only 1 company.
>
> It looks that the solution is to avoid locking in the sysrq case and
> trylock in the oops_in_progress case (see detailed analysis in
> 677fe555cbfb1).

Thanks for the detail information. the V2 patch didn't cover this case:

    stm32_usart_threaded_interrupt()
      spin_lock(&port->lock);
      ...
      uart_handle_sysrq_char();
        sysrq_function();
          printk();
            stm32_usart_console_write();
              locked = spin_trylock_irqsave(&port->lock); //better
than no lock(locked = 0) if other uart interrupt coming at this point

Find a same solution on fsl_lpuart.c
commit abf1e0a98083fd0a1069ce68ad8c92bfb97a57db

Thanks.

Best regards
Dillon
>
> So your initial patch looks to the right proposal, but it would be safer
> if Greg could confirm it.
>
> BR, Erwan.
>
>
> On 4/13/21 1:44 AM, 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.
> >
> > changes to below, should be more safe:
> >
> > .....
> > if (port->sysrq || oops_in_progress)
> >        locked = spin_trylock_irqsave(&port->lock, flags);
> > else
> >        spin_lock_irqsave(&port->lock, flags);
> >
> > ....
> >
> > if (locked)
> >       spin_unlock_irqrestore(&port->lock, flags);
> >
> > For current stm32 soc, it shouldn't happen. just a reminder for future.
> >
> > Thanks.
> >
> > Dillon
> >
> > On Mon, Apr 12, 2021 at 10:04 PM dillon min <dillon.minfei at gmail.com> wrote:
> >>
> >> Hi Johan,
> >>
> >> Yes, there is no deadlock. my fault.
> >> I forget the local_irq_save() plus spin_lock() is spin_lock_irqsave().
> >>
> >> Thanks for your review. please ignore this patch.
> >>
> >> Best regards
> >>
> >> Dillon
> >>
> >> On Mon, Apr 12, 2021 at 9:08 PM Johan Hovold <johan at kernel.org> wrote:
> >>>
> >>> On Mon, Apr 12, 2021 at 05:31:38PM +0800, dillon.minfei at gmail.com wrote:
> >>>> From: dillon min <dillon.minfei at gmail.com>
> >>>>
> >>>> To avoid potential deadlock in spin_lock usage, use spin_lock_irqsave,
> >>>> spin_trylock_irqsave(), spin_unlock_irqrestore() in process context.
> >>>
> >>> This doesn't make much sense as console_write can be called in any
> >>> context. And where's the deadlock you claim to be fixing here?
> >>>
> >>>> remove unused local_irq_save/restore call.
> >>>>
> >>>> Cc: Alexandre Torgue <alexandre.torgue at foss.st.com>
> >>>> Cc: Maxime Coquelin <mcoquelin.stm32 at gmail.com>
> >>>> Cc: Gerald Baeza <gerald.baeza at foss.st.com>
> >>>> Cc: Erwan Le Ray <erwan.leray at foss.st.com>
> >>>> Reported-by: kernel test robot <lkp at intel.com>
> >>>> Signed-off-by: dillon min <dillon.minfei at gmail.com>
> >>>> ---
> >>>> v2: remove unused code from stm32_usart_threaded_interrupt() according from
> >>>>      Greg's review.
> >>>>
> >>>>   drivers/tty/serial/stm32-usart.c | 8 +++-----
> >>>>   1 file changed, 3 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
> >>>> index b3675cf25a69..b1ba5e36e36e 100644
> >>>> --- a/drivers/tty/serial/stm32-usart.c
> >>>> +++ b/drivers/tty/serial/stm32-usart.c
> >>>> @@ -1354,13 +1354,12 @@ static void stm32_usart_console_write(struct console *co, const char *s,
> >>>>        u32 old_cr1, new_cr1;
> >>>>        int locked = 1;
> >>>>
> >>>> -     local_irq_save(flags);
> >>>>        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);
> >>>>
> >>>>        /* Save and disable interrupts, enable the transmitter */
> >>>>        old_cr1 = readl_relaxed(port->membase + ofs->cr1);
> >>>> @@ -1374,8 +1373,7 @@ static void stm32_usart_console_write(struct console *co, const char *s,
> >>>>        writel_relaxed(old_cr1, port->membase + ofs->cr1);
> >>>>
> >>>>        if (locked)
> >>>> -             spin_unlock(&port->lock);
> >>>> -     local_irq_restore(flags);
> >>>> +             spin_unlock_irqrestore(&port->lock, flags);
> >>>>   }
> >>>>
> >>>>   static int stm32_usart_console_setup(struct console *co, char *options)
> >>>
> >>> Johan