These helpers will be used when calling console->write_atomic on
KDB code in the next patch. It's basically the same implementation
as nbcon_device_try_acquire, but using NBCON_PRIO_EMERGENCY when
acquiring the context.

If the acquire succeeds, the message and message length are assigned to
nbcon_write_context so ->write_atomic can print the message.

After release try to flush the console since there may be a backlog of
messages in the ringbuffer. The kthread console printers do not get a
chance to run while kdb is active.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
 include/linux/console.h |  6 +++++
 kernel/printk/nbcon.c   | 66 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 72 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 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
+ * @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
+ *		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
+ *		using the given con->write_atomic() callback. Can be called
+ *		only when the console is usable at the moment.
+ */
+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);
+}

-- 
2.51.0

On 2025-09-30, Marcos Paulo de Souza <mpdesouza@suse.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.

> +}