linux-kernel - [PATCH 2/2] kdb: Adapt kdb_msg_write to work with NBCON consoles

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Message-Id: <20250713-nbcon-kgdboc-v1-2-51eccd9247a8@suse.com>
Date: Sun, 13 Jul 2025 02:09:19 -0300
From: Marcos Paulo de Souza <mpdesouza@...e.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 Petr Mladek <pmladek@...e.com>, 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>
Cc: linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net, 
 Marcos Paulo de Souza <mpdesouza@...e.com>
Subject: [PATCH 2/2] kdb: Adapt kdb_msg_write to work with NBCON consoles

Function kdb_msg_write was calling con->write for the any found console,
but it won't work on NBCON ones. 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.

Suggested-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.com>
---
 kernel/debug/kdb/kdb_io.c | 46 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 9b11b10b120cf07e451a7a4d92ce50f9a6c066b2..3b7365c11d06b01d487767fd89f1081da10dd2ed 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -558,6 +558,25 @@ static int kdb_search_string(char *searched, char *searchfor)
 	return 0;
 }
 
+static struct nbcon_context *nbcon_acquire_ctxt(struct console *con,
+					struct nbcon_write_context *wctxt,
+					char *msg, int msg_len)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	ctxt->console		    = con;
+	ctxt->spinwait_max_us	    = 0;
+	ctxt->prio		    = NBCON_PRIO_EMERGENCY;
+	ctxt->allow_unsafe_takeover = false;
+	wctxt->outbuf		    = msg;
+	wctxt->len		    = msg_len;
+
+	if (!nbcon_context_try_acquire(ctxt))
+		return NULL;
+
+	return ctxt;
+}
+
 static void kdb_msg_write(const char *msg, int msg_len)
 {
 	struct console *c;
@@ -589,12 +608,26 @@ 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))
+		struct nbcon_write_context wctxt = { };
+		struct nbcon_context *ctxt;
+		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;
+
+		/*
+		 * Do not continue if the console is NBCON and the context
+		 * can't be acquired.
+		 */
+		if (flags & CON_NBCON) {
+			ctxt = nbcon_acquire_ctxt(c, &wctxt, (char *)msg,
+						  msg_len);
+			if (!ctxt)
+				continue;
+		}
+
 		/*
 		 * Set oops_in_progress to encourage the console drivers to
 		 * disregard their internal spin locks: in the current calling
@@ -605,7 +638,12 @@ static void kdb_msg_write(const char *msg, int msg_len)
 		 * for this calling context.
 		 */
 		++oops_in_progress;
-		c->write(c, msg, msg_len);
+		if (flags & CON_NBCON) {
+			c->write_atomic(c, &wctxt);
+			nbcon_context_release(ctxt);
+		} else {
+			c->write(c, msg, msg_len);
+		}
 		--oops_in_progress;
 		touch_nmi_watchdog();
 	}

-- 
2.50.0