linux-kernel - Re: [PATCH v6 4/4] kdb: Switch to use safer dbg_io

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Date:   Fri, 5 Jun 2020 11:53:56 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     daniel.thompson@...aro.org, kgdb-bugreport@...ts.sourceforge.net,
        jason.wessel@...driver.com, dianders@...omium.org,
        sergey.senozhatsky@...il.com, gregkh@...uxfoundation.org,
        jslaby@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 4/4] kdb: Switch to use safer dbg_io_ops over console
 APIs

On Thu 2020-06-04 15:31:19, 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>

Looks good to me now:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr