linux-kernel - Re: [PATCH v4 4/4] kdb: Switch to use safer dbg_io

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Date:   Tue, 2 Jun 2020 14:32:41 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 4/4] kdb: Switch to use safer dbg_io_ops over console APIs

Hi,

On Fri, May 29, 2020 at 4:27 AM Sumit Garg <sumit.garg@...aro.org> wrote:
>
> In kgdb context, calling console handlers aren't safe due to locks used
> in those handlers which could in turn lead to a deadlock. Although, using
> oops_in_progress increases the chance to bypass locks in most console
> handlers but it might not be sufficient enough in case a console uses
> more locks (VT/TTY is good example).
>
> Currently when a driver provides both polling I/O and a console then kdb
> will output using the console. We can increase robustness by using the
> currently active polling I/O driver (which should be lockless) instead
> of the corresponding console. For several common cases (e.g. an
> embedded system with a single serial port that is used both for console
> output and debugger I/O) this will result in no console handler being
> used.
>
> In order to achieve this we need to reverse the order of preference to
> use dbg_io_ops (uses polling I/O mode) over console APIs. So we just
> store "struct console" that represents debugger I/O in dbg_io_ops and
> while emitting kdb messages, skip console that matches dbg_io_ops
> console in order to avoid duplicate messages. After this change,
> "is_console" param becomes redundant and hence removed.
>
> Suggested-by: Daniel Thompson <daniel.thompson@...aro.org>
> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> ---
>  drivers/tty/serial/kgdb_nmi.c | 2 +-
>  drivers/tty/serial/kgdboc.c   | 4 ++--

I don't think this will compile against the "kgdboc_earlycon" patches
that landed, will it?  Specifically when I apply your patch I still
see "is_console" in:

static struct kgdb_io kgdboc_earlycon_io_ops = {
  .name = "kgdboc_earlycon",
  .read_char = kgdboc_earlycon_get_char,
  .write_char = kgdboc_earlycon_put_char,
  .pre_exception = kgdboc_earlycon_pre_exp_handler,
  .deinit = kgdboc_earlycon_deinit,
  .is_console = true,
};


> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index b072aeb..bc0face3 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -273,8 +273,7 @@ struct kgdb_arch {
>   * the I/O driver.
>   * @post_exception: Pointer to a function that will do any cleanup work
>   * for the I/O driver.
> - * @is_console: 1 if the end device is a console 0 if the I/O device is
> - * not a console
> + * @cons: valid if the I/O device is a console.

optional nit: add "; else NULL"


Other than that this looks great.  Feel free to add my Reviewed-by:
tag once you've fixed the error that the bot found and resolved with
kgdb_earlycon.


-Doug