linux-kernel - Re: [PATCH v4 5/5] kdb: Adapt kdb_msg

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Message-ID: <aMq1GRHwo6xnsPBG@pathway.suse.cz>
Date: Wed, 17 Sep 2025 15:18:17 +0200
From: Petr Mladek <pmladek@...e.com>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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 v4 5/5] kdb: Adapt kdb_msg_write to work with NBCON
 consoles

On Mon 2025-09-15 08:20:34, Marcos Paulo de Souza wrote:
> Function kdb_msg_write was calling con->write for any found console,
> but it won't work on NBCON consoles. 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.
> 
> 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.
> 
> The oops_in_progress handling is only needed in the legacy consoles,
> so it was moved around the con->write callback.
> 
> Suggested-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.com>

It looks good to me:

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

See one note below.

> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -589,24 +589,41 @@ 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))
> +		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;
> -		/*
> -		 * Set oops_in_progress to encourage the console drivers to
> -		 * disregard their internal spin locks: in the current calling
> -		 * context the risk of deadlock is a bigger problem than risks
> -		 * due to re-entering the console driver. We operate directly on
> -		 * oops_in_progress rather than using bust_spinlocks() because
> -		 * the calls bust_spinlocks() makes on exit are not appropriate
> -		 * for this calling context.
> -		 */
> -		++oops_in_progress;
> -		c->write(c, msg, msg_len);
> -		--oops_in_progress;
> +
> +		if (flags & CON_NBCON) {
> +			struct nbcon_write_context wctxt = { };
> +
> +			/*
> +			 * Do not continue if the console is NBCON and the context
> +			 * can't be acquired.
> +			 */
> +			if (!nbcon_kdb_try_acquire(c, &wctxt))
> +				continue;
> +
> +			nbcon_write_context_set_buf(&wctxt, (char *)msg, msg_len);

I have overlooked the (char *) cast in the earlier versions of the
patchset. It would be nice to fix the nbcon API so that the parameter
could be passed as (const char *). It looks that the API using
struct nbcon_write_context never modifies the given buffer so
it would be the right thing.

But it is beyond the scope of this patchset. It would be material
for a separate code clean up ;-)

Best Regards,
Petr