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

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Message-ID: <CAL9mu0Lt-3_O7V5HLxd5Hbt9afx9ryBUzWqmsc+2n3SP7JS6ig@mail.gmail.com>
Date:   Mon, 12 Apr 2021 14:50:20 +0800
From:   dillon min <dillon.minfei@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     jirislaby@...nel.org, Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...s.st.com>,
        linux-serial@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] serial: stm32: optimize spin lock usage

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@...uxfoundation.org> wrote:
>
> On Mon, Apr 12, 2021 at 12:34:21PM +0800, dillon.minfei@...il.com wrote:
> > From: dillon min <dillon.minfei@...il.com>
> >
> > 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@...il.com>
> > ---
> > 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 ?

There is a bug in this patch, the variable flags should be a global variable.

Thanks.

Dillon,
>
> thanks,
>
> greg k-h