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 [off-list ref] wrote:

quoted

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 [off-list ref] wrote:

quoted

On Mon, Apr 12, 2021 at 05:31:38PM +0800, dillon.minfei@gmail.com wrote:

quoted

From: dillon min <redacted>

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?

quoted

remove unused local_irq_save/restore call.

Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Gerald Baeza <redacted>
Cc: Erwan Le Ray <redacted>
Reported-by: kernel test robot <redacted>
Signed-off-by: dillon min <redacted>
---
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

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