linux-kernel - Re: [PATCH v2 2/3] printk: nbcon: Introduce KDB helpers

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Message-ID: <84jz2qkve0.fsf@jogness.linutronix.de>
Date: Tue, 26 Aug 2025 16:36:47 +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 v2 2/3] printk: nbcon: Introduce KDB helpers

On 2025-08-11, Marcos Paulo de Souza <mpdesouza@...e.com> wrote:
> These helpers will be used when calling console->write_atomic on
> KDB code in the next patch. It's basically the same implementaion
> as nbcon_device_try_acquire, but using NBCON_PORIO_EMERGENCY when
> acquiring the context.
>
> For release, differently from nbcon_device_release, we don't need to
> flush the console, since all CPUs are stopped when KDB is active.
>
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.com>
> ---
>  include/linux/console.h |  6 ++++++
>  kernel/printk/nbcon.c   | 26 ++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
>
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 67af483574727c00eea1d5a1eacc994755c92607..b34c5a0b86303e2fb4583fa467d8be43761cf756 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -605,6 +605,9 @@ extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt);
>  extern bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt);
>  extern bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt);
>  extern void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt);
> +extern bool nbcon_kdb_try_acquire(struct console *con,
> +				  struct nbcon_write_context *wctxt);
> +extern void nbcon_kdb_release(struct nbcon_write_context *wctxt);
>  
>  /*
>   * Check if the given console is currently capable and allowed to print
> @@ -654,6 +657,9 @@ static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return
>  static inline bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt) { return false; }
>  static inline bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt) { return false; }
>  static inline void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { }
> +static inline bool nbcon_kdb_try_acquire(struct console *con,
> +					 struct nbcon_write_context *wctxt) { return false; }
> +static inline void nbcon_kdb_release(struct console *con) { }
>  static inline bool console_is_usable(struct console *con, short flags,
>  				     bool use_atomic) { return false; }
>  #endif
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 646801813415f0abe40cabf2f28ca9e30664f028..79d8c7437806119ad9787ddc48382dc2c86c23c3 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1855,3 +1855,29 @@ void nbcon_device_release(struct console *con)
>  	console_srcu_read_unlock(cookie);
>  }
>  EXPORT_SYMBOL_GPL(nbcon_device_release);
> +
> +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;
> +}
> +
> +void nbcon_kdb_release(struct nbcon_write_context *wctxt)
> +{
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +
> +	nbcon_context_exit_unsafe(ctxt);
> +	nbcon_context_release(ctxt);

If nbcon_context_exit_unsafe() fails, the current task is no longer the
owner and thus a release is not needed. I would prefer:

	if (nbcon_context_exit_unsafe(ctxt))
		nbcon_context_release(ctxt);

or

	if (!nbcon_context_exit_unsafe(ctxt))
		return;

	nbcon_context_release(ctxt);

You can find this same pattern in nbcon_device_release().

John