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From:   Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Date:   Mon, 2 Dec 2019 09:37:41 -0700
Message-ID: <CAOCk7NryqvGjkWYKcHp+bNxqfG7AWsN5tvhx71ZWObvssXV3mA@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] tty: serial: msm_serial: Fix lockup for sysrq and oops
To:     Leo Yan <leo.yan@linaro.org>
Cc:     Andy Gross <agross@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Nicolas Dechesne <nicolas.dechesne@linaro.org>,
        MSM <linux-arm-msm@vger.kernel.org>,
        linux-serial@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Wed, Nov 27, 2019 at 7:16 AM Leo Yan <leo.yan@linaro.org> wrote:
>
> As the commit 677fe555cbfb ("serial: imx: Fix recursive locking bug")
> has mentioned the uart driver might cause recursive locking between
> normal printing and the kernel debugging facilities (e.g. sysrq and
> oops).  In the commit it gave out suggestion for fixing recursive
> locking issue: "The solution is to avoid locking in the sysrq case
> and trylock in the oops_in_progress case."
>
> This patch follows the suggestion (also used the exactly same code with
> other serial drivers, e.g. amba-pl011.c) to fix the recursive locking
> issue, this can avoid stuck caused by deadlock and print out log for
> sysrq and oops.
>
> Fixes: 04896a77a97b ("msm_serial: serial driver for MSM7K onboard serial peripheral.")
> Signed-off-by: Leo Yan <leo.yan@linaro.org>

Looks sane to me
Reviewed-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>

> ---
>  drivers/tty/serial/msm_serial.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
> index 3657a24913fc..889538182e83 100644
> --- a/drivers/tty/serial/msm_serial.c
> +++ b/drivers/tty/serial/msm_serial.c
> @@ -1576,6 +1576,7 @@ static void __msm_console_write(struct uart_port *port, const char *s,
>         int num_newlines = 0;
>         bool replaced = false;
>         void __iomem *tf;
> +       int locked = 1;
>
>         if (is_uartdm)
>                 tf = port->membase + UARTDM_TF;
> @@ -1588,7 +1589,13 @@ static void __msm_console_write(struct uart_port *port, const char *s,
>                         num_newlines++;
>         count += num_newlines;
>
> -       spin_lock(&port->lock);
> +       if (port->sysrq)
> +               locked = 0;
> +       else if (oops_in_progress)
> +               locked = spin_trylock(&port->lock);
> +       else
> +               spin_lock(&port->lock);
> +
>         if (is_uartdm)
>                 msm_reset_dm_count(port, count);
>
> @@ -1624,7 +1631,9 @@ static void __msm_console_write(struct uart_port *port, const char *s,
>                 iowrite32_rep(tf, buf, 1);
>                 i += num_chars;
>         }
> -       spin_unlock(&port->lock);
> +
> +       if (locked)
> +               spin_unlock(&port->lock);
>  }
>
>  static void msm_console_write(struct console *co, const char *s,
> --
> 2.17.1
>