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From:   Doug Anderson <dianders@chromium.org>
Date:   Thu, 5 Dec 2019 08:40:20 +0800
Message-ID: <CAD=FV=W2nENJF0fNpTzjuAVOo_AoZQryThua9vdtt-zsMk82qg@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>,
        Jeffrey Hugo <jeffrey.l.hugo@gmail.com>,
        linux-arm-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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Hi,

On Wed, Nov 27, 2019 at 10:16 PM 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>
> ---
>  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);

I don't have tons of experience with the "msm" serial driver, but the
above snippet tickled a memory in my brain for when I was looking at
the "qcom_geni" serial driver, which is a close cousin.

I seemed to remember that the "if (port->sysrq)" was something you
didn't want.  ...but maybe that's only if you do something like commit
336447b3298c ("serial: qcom_geni_serial: Process sysrq at port unlock
time")?  Any way you can try making a similar change to the msm driver
and see if it allow you to remove the special case for "port->sysrq"?

-Doug