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

[Erwan LE RAY]

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.

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).

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@xxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > On Mon, Apr 12, 2021 at 05:31:38PM +0800, dillon.minfei@xxxxxxxxx wrote:
> > > > From: dillon min <dillon.minfei@xxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxx>
> > > > Cc: Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>
> > > > Cc: Gerald Baeza <gerald.baeza@xxxxxxxxxxx>
> > > > Cc: Erwan Le Ray <erwan.leray@xxxxxxxxxxx>
> > > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > > Signed-off-by: dillon min <dillon.minfei@xxxxxxxxx>
> > > > ---
> > > > 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