Re: [Linux-stm32] [PATCH] serial: stm32: optimize spin lock usage

[Erwan LE RAY]

Hi Dillon,

Thanks for your patch.

Could you please elaborate the use case in your commit message ?

Best Regards, Erwan.

On 4/12/21 10:54 AM, dillon min wrote:
> Hi Greg,
>
> On Mon, Apr 12, 2021 at 4:25 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Mon, Apr 12, 2021 at 02:50:20PM +0800, dillon min wrote:
> > > Hi Greg，
> > >
> > > Thanks for the quick response, please ignore the last private mail.
> > >
> > > On Mon, Apr 12, 2021 at 1:52 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > > On Mon, Apr 12, 2021 at 12:34:21PM +0800, dillon.minfei@xxxxxxxxx wrote:
> > > > > From: dillon min <dillon.minfei@xxxxxxxxx>
> > > > >
> > > > > To avoid potential deadlock in spin_lock usage, change to use
> > > > > spin_lock_irqsave(), spin_unlock_irqrestore() in process(thread_fn) context.
> > > > > spin_lock(), spin_unlock() under handler context.
> > > > >
> > > > > remove unused local_irq_save/restore call.
> > > > >
> > > > > Signed-off-by: dillon min <dillon.minfei@xxxxxxxxx>
> > > > > ---
> > > > > Was verified on stm32f469-disco board. need more test on stm32mp platform.
> > > > >
> > > > >   drivers/tty/serial/stm32-usart.c | 27 +++++++++++++++++----------
> > > > >   1 file changed, 17 insertions(+), 10 deletions(-)
> > > > >
> > > > > diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
> > > > > index b3675cf25a69..c4c859b34367 100644
> > > > > --- a/drivers/tty/serial/stm32-usart.c
> > > > > +++ b/drivers/tty/serial/stm32-usart.c
> > > > > @@ -214,7 +214,7 @@ static void stm32_usart_receive_chars(struct uart_port *port, bool threaded)
> > > > >        struct tty_port *tport = &port->state->port;
> > > > >        struct stm32_port *stm32_port = to_stm32_port(port);
> > > > >        const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
> > > > > -     unsigned long c;
> > > > > +     unsigned long c, flags;
> > > > >        u32 sr;
> > > > >        char flag;
> > > > >
> > > > > @@ -276,9 +276,17 @@ static void stm32_usart_receive_chars(struct uart_port *port, bool threaded)
> > > > >                uart_insert_char(port, sr, USART_SR_ORE, c, flag);
> > > > >        }
> > > > >
> > > > > -     spin_unlock(&port->lock);
> > > > > +     if (threaded)
> > > > > +             spin_unlock_irqrestore(&port->lock, flags);
> > > > > +     else
> > > > > +             spin_unlock(&port->lock);
> > > >
> > > > You shouldn't have to check for this, see the other patches on the list
> > > > recently that fixed this up to not be an issue for irq handlers.
> > > Can you help to give more hints, or the commit id of the patch which
> > > fixed this. thanks.
> > >
> > > I'm still confused with this.
> > >
> > > The stm32_usart_threaded_interrupt() is a kthread context, once
> > > port->lock holds by this function, another serial interrupts raised,
> > > such as USART_SR_TXE,stm32_usart_interrupt() can't get the lock,
> > > there will be a deadlock. isn't it?
> > >
> > >   So, shouldn't I use spin_lock{_irqsave} according to the caller's context ?
> >
> > Please see 81e2073c175b ("genirq: Disable interrupts for force threaded
> > handlers") for when threaded irq handlers have irqs disabled, isn't that
> > the case you are trying to "protect" from here?
> >
> > Why is the "threaded" flag used at all?  The driver should not care.
> >
> > Also see 9baedb7baeda ("serial: imx: drop workaround for forced irq
> > threading") in linux-next for an example of how this was fixed up in a
> > serial driver.
> >
> > does that help?
> Yes, it's really helpful. and 81e2073c175b should be highlighted in a doc.
> In my past knowledge, we should care about hard irq & thread_fn lock conflict.
> This patch has totally avoided patching code in the separate driver side.
> thanks.
>
> I will just keep the changes in stm32_usart_console_write(), remove
> these code in
> thread_fn. update version 2 for you.
>
> thanks.
>
> Dillon,
> > thanks,
> >
> > greg k-h
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