linux-kernel - Re: [PATCH v2 3/3] kdb: Adapt kdb_msg

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Message-ID: <aJoAYD_r7ygH9AdS@aspen.lan>
Date: Mon, 11 Aug 2025 15:38:24 +0100
From: Daniel Thompson <daniel@...cstar.com>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Petr Mladek <pmladek@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Jason Wessel <jason.wessel@...driver.com>,
	Daniel Thompson <danielt@...nel.org>,
	Douglas Anderson <dianders@...omium.org>,
	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH v2 3/3] kdb: Adapt kdb_msg_write to work with NBCON
 consoles

Hi Marcos

No objections, but a couple of questions if I may...


On Mon, Aug 11, 2025 at 10:32:47AM -0300, Marcos Paulo de Souza wrote:
> Function kdb_msg_write was calling con->write for any found console,
> but it won't work on NBCON ones. In this case we should acquire the
> ownership of the console using NBCON_PRIO_EMERGENCY, since printing
> kdb messages should only be interrupted by a panic. This is done by the
> nbcon_kdb_{acquire,release} functions.

Just wanted to check what it means to be "interrupted by a panic"?

kdb is called from the panic handler but, assuming the serial port is run
syncrhonously when "bad things are happening", the serial port should be
quiet when we enter kdb meaning we can still acquire ownership of the
console?
>
> At this point, the console is required to use the atomic callback. The
> console is skipped if the write_atomic callback is not set or if the
> context could not be acquired. The validation of NBCON is done by the
> console_is_usable helper. The context is released right after
> write_atomic finishes.
>
> Suggested-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.com>
> ---
>  kernel/debug/kdb/kdb_io.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 9b11b10b120cf07e451a7a4d92ce50f9a6c066b2..74f6d4316bdc9d3c4f6d4252bf425e33cce65a87 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -589,12 +589,23 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  	 */
>  	cookie = console_srcu_read_lock();
>  	for_each_console_srcu(c) {
> -		if (!(console_srcu_read_flags(c) & CON_ENABLED))
> +		struct nbcon_write_context wctxt = { };
> +		short flags = console_srcu_read_flags(c);
> +
> +		if (!console_is_usable(c, flags, true))
>  			continue;
>  		if (c == dbg_io_ops->cons)
>  			continue;
> -		if (!c->write)
> -			continue;
> +
> +		/*
> +		 * Do not continue if the console is NBCON and the context
> +		 * can't be acquired.
> +		 */
> +		if (flags & CON_NBCON) {
> +			if (!nbcon_kdb_try_acquire(c, &wctxt))
> +				continue;
> +		}
> +
>  		/*
>  		 * Set oops_in_progress to encourage the console drivers to
>  		 * disregard their internal spin locks: in the current calling
> @@ -605,7 +616,14 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  		 * for this calling context.
>  		 */
>  		++oops_in_progress;

Dumb question, but is the oops_in_progress bump still useful with atomic
consoles? Will the console have any spinlocks to disregard or will the
console ownership code already handled any mutual exclusion issues meaning
there should be no spinlocks taking by the atomic write handler?


Thanks

Daniel.