netconsole: convert to NBCON console infrastructure

Convert netconsole from the legacy console API to the NBCON framework.
NBCON provides threaded printing which unblocks printk()s and flushes in
a thread, decoupling network TX from printk() when netconsole is
in use.

Since netconsole relies on the network stack which cannot safely operate
from all atomic contexts, mark both consoles with
CON_NBCON_ATOMIC_UNSAFE. (See discussion in [1])

CON_NBCON_ATOMIC_UNSAFE restricts write_atomic() usage to emergency
scenarios (panic) where regular messages are sent in threaded mode.

Implementation changes:
- Unify write_ext_msg() and write_msg() into netconsole_write()
- Add device_lock/device_unlock callbacks to manage target_list_lock
- Use nbcon_enter_unsafe()/nbcon_exit_unsafe() around network
  operations.
  - If nbcon_enter_unsafe() fails, just return given netconsole lost
    the ownership of the console.
- Set write_thread and write_atomic callbacks (both use same function)

Link: https://lore.kernel.org/all/b2qps3uywhmjaym4mht2wpxul4yqtuuayeoq4iv4k3zf5wdgh3@tocu6c7mj4lt/ [1]
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260206-nbcon-v7-3-62bda69b1b41@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/netconsole.c

@@ -1859,23 +1859,6 @@ static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
 				   sysdata_len);
 }
 
-static void write_ext_msg(struct console *con, const char *msg,
-			  unsigned int len)
-{
-	struct netconsole_target *nt;
-	unsigned long flags;
-
-	if ((oops_only && !oops_in_progress) || list_empty(&target_list))
-		return;
-
-	spin_lock_irqsave(&target_list_lock, flags);
-	list_for_each_entry(nt, &target_list, list)
-		if (nt->extended && nt->state == STATE_ENABLED &&
-		    netif_running(nt->np.dev))
-			send_ext_msg_udp(nt, msg, len);
-	spin_unlock_irqrestore(&target_list_lock, flags);
-}
-
 static void send_msg_udp(struct netconsole_target *nt, const char *msg,
 			 unsigned int len)
 {
@@ -1890,30 +1873,64 @@ static void send_msg_udp(struct netconsole_target *nt, const char *msg,
 	}
 }
 
-static void write_msg(struct console *con, const char *msg, unsigned int len)
+/**
+ * netconsole_write - Generic function to send a msg to all targets
+ * @wctxt: nbcon write context
+ * @extended: "true" for extended console mode
+ *
+ * Given an nbcon write context, send the message to the netconsole targets
+ */
+static void netconsole_write(struct nbcon_write_context *wctxt, bool extended)
 {
-	unsigned long flags;
 	struct netconsole_target *nt;
 
 	if (oops_only && !oops_in_progress)
 		return;
-	/* Avoid taking lock and disabling interrupts unnecessarily */
+
-	if (list_empty(&target_list))
+	list_for_each_entry(nt, &target_list, list) {
+		if (nt->extended != extended || nt->state != STATE_ENABLED ||
+		    !netif_running(nt->np.dev))
+			continue;
+
+		/* If nbcon_enter_unsafe() fails, just return given netconsole
+		 * lost the ownership, and iterating over the targets will not
+		 * be able to re-acquire.
+		 */
+		if (!nbcon_enter_unsafe(wctxt))
 			return;
 
-	spin_lock_irqsave(&target_list_lock, flags);
+		if (extended)
-	list_for_each_entry(nt, &target_list, list) {
+			send_ext_msg_udp(nt, wctxt->outbuf, wctxt->len);
-		if (!nt->extended && nt->state == STATE_ENABLED &&
+		else
-		    netif_running(nt->np.dev)) {
+			send_msg_udp(nt, wctxt->outbuf, wctxt->len);
-			/*
+
-			 * We nest this inside the for-each-target loop above
+		nbcon_exit_unsafe(wctxt);
-			 * so that we're able to get as much logging out to
-			 * at least one target if we die inside here, instead
-			 * of unnecessarily keeping all targets in lock-step.
-			 */
-			send_msg_udp(nt, msg, len);
-		}
 	}
+}
+
+static void netconsole_write_ext(struct console *con __always_unused,
+				 struct nbcon_write_context *wctxt)
+{
+	netconsole_write(wctxt, true);
+}
+
+static void netconsole_write_basic(struct console *con __always_unused,
+				   struct nbcon_write_context *wctxt)
+{
+	netconsole_write(wctxt, false);
+}
+
+static void netconsole_device_lock(struct console *con __always_unused,
+				   unsigned long *flags)
+__acquires(&target_list_lock)
+{
+	spin_lock_irqsave(&target_list_lock, *flags);
+}
+
+static void netconsole_device_unlock(struct console *con __always_unused,
+				     unsigned long flags)
+__releases(&target_list_lock)
+{
 	spin_unlock_irqrestore(&target_list_lock, flags);
 }
 
@@ -2078,14 +2095,20 @@ static void free_param_target(struct netconsole_target *nt)
 
 static struct console netconsole_ext = {
 	.name = "netcon_ext",
-	.flags	= CON_ENABLED | CON_EXTENDED,
+	.flags = CON_ENABLED | CON_EXTENDED | CON_NBCON | CON_NBCON_ATOMIC_UNSAFE,
-	.write	= write_ext_msg,
+	.write_thread = netconsole_write_ext,
+	.write_atomic = netconsole_write_ext,
+	.device_lock = netconsole_device_lock,
+	.device_unlock = netconsole_device_unlock,
 };
 
 static struct console netconsole = {
 	.name = "netcon",
-	.flags	= CON_ENABLED,
+	.flags = CON_ENABLED | CON_NBCON | CON_NBCON_ATOMIC_UNSAFE,
-	.write	= write_msg,
+	.write_thread = netconsole_write_basic,
+	.write_atomic = netconsole_write_basic,
+	.device_lock = netconsole_device_lock,
+	.device_unlock = netconsole_device_unlock,
 };
 
 static int __init init_netconsole(void)