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

[dillon min]

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