[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 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