linux-kernel - [PATCH v3 4/4] kdb: Adapt kdb_msg_write to work with NBCON consoles

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Message-Id: <20250902-nbcon-kgdboc-v3-4-cd30a8106f1c@suse.com>
Date: Tue, 02 Sep 2025 15:33:55 -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 v3 4/4] kdb: Adapt kdb_msg_write to work with NBCON
 consoles

Function kdb_msg_write was calling con->write for 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 in the case it was
triggered by sysrq debug option. This is done by the
nbcon_kdb_{acquire,release} functions.

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.

The oops_in_progress handling is only needed in the legacy consoles,
so it was moved around the con->write callback.

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, 31 insertions(+), 15 deletions(-)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 9b11b10b120cf07e451a7a4d92ce50f9a6c066b2..47bc31cc71bc84750db5d9304ed75a113cd382bf 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -589,24 +589,40 @@ 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 = { };
+		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;
-		/*
-		 * Set oops_in_progress to encourage the console drivers to
-		 * disregard their internal spin locks: in the current calling
-		 * context the risk of deadlock is a bigger problem than risks
-		 * due to re-entering the console driver. We operate directly on
-		 * oops_in_progress rather than using bust_spinlocks() because
-		 * the calls bust_spinlocks() makes on exit are not appropriate
-		 * for this calling context.
-		 */
-		++oops_in_progress;
-		c->write(c, msg, msg_len);
-		--oops_in_progress;
+
+		if (flags & CON_NBCON) {
+			/*
+			 * Do not continue if the console is NBCON and the context
+			 * can't be acquired.
+			 */
+			if (!nbcon_kdb_try_acquire(c, &wctxt))
+				continue;
+
+			wctxt.outbuf = (char *)msg;
+			wctxt.len = msg_len;
+			c->write_atomic(c, &wctxt);
+			nbcon_kdb_release(&wctxt);
+		} else {
+			/*
+			 * Set oops_in_progress to encourage the console drivers to
+			 * disregard their internal spin locks: in the current calling
+			 * context the risk of deadlock is a bigger problem than risks
+			 * due to re-entering the console driver. We operate directly on
+			 * oops_in_progress rather than using bust_spinlocks() because
+			 * the calls bust_spinlocks() makes on exit are not appropriate
+			 * for this calling context.
+			 */
+			++oops_in_progress;
+			c->write(c, msg, msg_len);
+			--oops_in_progress;
+		}
 		touch_nmi_watchdog();
 	}
 	console_srcu_read_unlock(cookie);

-- 
2.50.0