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

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Date:   Tue, 2 Jun 2020 19:32:46 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        jslaby@...e.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 4/4] kdb: Switch to use safer dbg_io_ops over console APIs

On Tue, 2 Jun 2020 at 19:16, Daniel Thompson <daniel.thompson@...aro.org> wrote:
>
> On Fri, May 29, 2020 at 04:56:47PM +0530, Sumit Garg 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>
>
> Looking good, only one minor comment left on my side (including the
> three patches prior).
>
> > diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> > index 9e5a40d..5e00bc8 100644
> > --- a/kernel/debug/kdb/kdb_io.c
> > +++ b/kernel/debug/kdb/kdb_io.c
> > @@ -560,12 +560,14 @@ static void kdb_msg_write(char *msg, int msg_len)
> >       if (msg_len == 0)
> >               return;
> >
> > -     if (dbg_io_ops && !dbg_io_ops->is_console)
> > +     if (dbg_io_ops)
> >               kdb_io_write(msg, msg_len);
>
> Since this now slots on so cleanly and there are not multiple calls
> to kdb_io_write() then I think perhaps factoring this out into its
> own function (in patch 1) is no long necessary. The character write
> loop can go directly into this function.
>

Okay, will update it in the next version.

-Sumit

>
> Daniel.