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>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
 include/linux/console.h |  6 +++++
 kernel/printk/nbcon.c   | 61 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index 1fda485ec2fa..0d589a933b6b 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -606,6 +606,9 @@ 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
@@ -655,6 +658,9 @@ 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 558ef3177976..e1bf5409cb6b 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1855,3 +1855,64 @@ 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 and enter unsafe
+ *			   section
+ * @con:	The nbcon console to acquire
+ * @wctxt:	The nbcon write context to be used on success
+ *
+ * Context:	Under console_srcu_read_lock() for emitting 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()
+ */
+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 Thu 2025-10-16 11:47:55, Marcos Paulo de Souza wrote:
> 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.
> 
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -606,6 +606,9 @@ 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
> @@ -655,6 +658,9 @@ 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) { }

A test robot found that this definition is using a wrong type,
see https://lore.kernel.org/all/202510171023.YREXxhMK-lkp@intel.com/

It should be:

static inline void nbcon_kdb_release(struct nbcon_write_context *wctxt) { }

No need for v7. I am going to fix this when committing the patchset.

Best Regards,
Petr