linux-kernel - Re: [PATCH v5 2/5] printk: nbcon: Introduce KDB helpers

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Message-ID: <84h5wihdqu.fsf@jogness.linutronix.de>
Date: Wed, 01 Oct 2025 17:02:41 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Marcos Paulo de Souza <mpdesouza@...e.com>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Petr Mladek <pmladek@...e.com>, Steven
 Rostedt <rostedt@...dmis.org>, Sergey Senozhatsky
 <senozhatsky@...omium.org>, Jason Wessel <jason.wessel@...driver.com>,
 Daniel Thompson <danielt@...nel.org>, Douglas Anderson
 <dianders@...omium.org>
Cc: linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
 Marcos Paulo de Souza <mpdesouza@...e.com>
Subject: Re: [PATCH v5 2/5] printk: nbcon: Introduce KDB helpers

On 2025-09-30, Marcos Paulo de Souza <mpdesouza@...e.com> wrote:
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 558ef31779760340ce42608294d91d5401239f1d..c23abed5933527cb7c6bcc42057fadbb44a43446 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1855,3 +1855,69 @@ void nbcon_device_release(struct console *con)
>  	console_srcu_read_unlock(cookie);
>  }
>  EXPORT_SYMBOL_GPL(nbcon_device_release);
> +
> +/**
> + * nbcon_kdb_try_acquire - Try to acquire nbcon console, enter unsafe
> + *				section, and initialized nbcon write context

                               initialize ---^^^^^^^^^^^

And technically it is not initializing the write context, just the
console ownership context. I'm not sure it is really necessary to
mention that.

> + * @con:	The nbcon console to acquire
> + * @wctxt:	The nbcon write context to be used on success
> + *
> + * Context:	Under console_srcu_read_lock() for emiting a single kdb message

                                       emitting ---^^^^^^^

"./scripts/checkpatch.pl --codespell" is your friend. ;-)

> + *		using the given con->write_atomic() callback. Can be called
> + *		only when the console is usable at the moment.
> + *
> + * Return:	True if the console was acquired. False otherwise.
> + *
> + * kdb emits messages on consoles registered for printk() without
> + * storing them into the ring buffer. It has to acquire the console
> + * ownerhip so that it could call con->write_atomic() callback a safe way.
> + *
> + * This function acquires the nbcon console using priority NBCON_PRIO_EMERGENCY
> + * and marks it unsafe for handover/takeover.
> + */
> +bool nbcon_kdb_try_acquire(struct console *con,
> +			   struct nbcon_write_context *wctxt)
> +{
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +
> +	memset(ctxt, 0, sizeof(*ctxt));
> +	ctxt->console = con;
> +	ctxt->prio    = NBCON_PRIO_EMERGENCY;
> +
> +	if (!nbcon_context_try_acquire(ctxt, false))
> +		return false;
> +
> +	if (!nbcon_context_enter_unsafe(ctxt))
> +		return false;
> +
> +	return true;
> +}
> +
> +/**
> + * nbcon_kdb_release - Exit unsafe section and release the nbcon console
> + *
> + * @wctxt:	The nbcon write context initialized by a successful
> + *		nbcon_kdb_try_acquire()
> + *
> + * Context:	Under console_srcu_read_lock() for emiting a single kdb message

                                       emitting ---^^^^^^^

> + *		using the given con->write_atomic() callback. Can be called
> + *		only when the console is usable at the moment.

I do not think the "Context" is relevant. It must be called if
a previous call to nbcon_kdb_try_acquire() was successful.

> + */
> +void nbcon_kdb_release(struct nbcon_write_context *wctxt)
> +{
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +
> +	if (!nbcon_context_exit_unsafe(ctxt))
> +		return;
> +
> +	nbcon_context_release(ctxt);
> +
> +	/*
> +	 * Flush any new printk() messages added when the console was blocked.
> +	 * Only the console used by the given write context was	blocked.
> +	 * The console was locked only when the write_atomic() callback
> +	 * was usable.
> +	 */
> +	__nbcon_atomic_flush_pending_con(ctxt->console,
> +					 prb_next_reserve_seq(prb), false);

This can all be one line. 100 characters is the official limit for code.

> +}