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

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Message-ID: <CAFA6WYPfxMtykUES3MqhK++WC7EkfDdQRbMPsoEqPcdx41O6jA@mail.gmail.com>
Date:   Wed, 3 Jun 2020 10:19:11 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Doug Anderson <dianders@...omium.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

On Wed, 3 Jun 2020 at 03:02, Doug Anderson <dianders@...omium.org> wrote:
>
> 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:

Agree will fix this and rebase this patch-set onto Daniel's tree.

>
> 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"
>

Okay.

>
> 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.
>

Thanks.

-Sumit

>
> -Doug