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

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Date:   Tue, 2 Jun 2020 14:46:50 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net, jason.wessel@...driver.com,
        dianders@...omium.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, gregkh@...uxfoundation.org,
        jslaby@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/4] kdb: Switch to use safer dbg_io_ops over console
 APIs

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.


Daniel.