Messages in this thread | | | Subject | Re: [PATCH v2] serial: stm32: optimize spin lock usage | From | Erwan LE RAY <> | Date | Thu, 15 Apr 2021 19:09:14 +0200 |
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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@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@kernel.org> wrote: >>> >>> On Mon, Apr 12, 2021 at 05:31:38PM +0800, dillon.minfei@gmail.com wrote: >>>> From: dillon min <dillon.minfei@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@foss.st.com> >>>> Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com> >>>> Cc: Gerald Baeza <gerald.baeza@foss.st.com> >>>> Cc: Erwan Le Ray <erwan.leray@foss.st.com> >>>> Reported-by: kernel test robot <lkp@intel.com> >>>> Signed-off-by: dillon min <dillon.minfei@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
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