printk: Allow to use the printk kthread immediately even for 1st nbcon

The kthreads for nbcon consoles are created by nbcon_alloc() at
the beginning of the console registration. But it currently works
only for the 2nd or later nbcon console because the code checks
@printk_kthreads_running.

The kthread for the 1st registered nbcon console is created at the very
end of register_console() by printk_kthreads_check_locked(). As a result,
the entire log is replayed synchronously when the "enabled" message
gets printed. It might block the boot for a long time with a slow serial
console.

Prevent the synchronous flush by creating the kthread even for the 1st
nbcon console when it is safe (kthreads ready and no boot consoles).

Also inform printk() to use the kthread by setting
@printk_kthreads_running. Note that the kthreads already must be
running when it is safe and this is not the 1st nbcon console.

Symmetrically, clear @printk_kthreads_running when the last nbcon
console was unregistered by nbcon_free(). This requires updating
@have_nbcon_console before nbcon_free() gets called.

Note that there is _no_ problem when the 1st nbcon console replaces boot
consoles. In this case, the kthread will be started at the end
of registration after the boot consoles are removed. But the console
does not reply the entire log buffer in this case. Note that
the flag CON_PRINTBUFFER is always cleared when the boot consoles are
removed and vice versa.

Closes: https://lore.kernel.org/r/20250514173514.2117832-1-mcobb@thegoodpenguin.co.uk
Tested-by: Michael Cobb <mcobb@thegoodpenguin.co.uk>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20250604142045.253301-1-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 24 insertions, 2 deletions

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index fd12efcc4aed..d60596777d27 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1671,6 +1671,9 @@ bool nbcon_alloc(struct console *con)
 {
 	struct nbcon_state state = { };
 
+	/* Synchronize the kthread start. */
+	lockdep_assert_console_list_lock_held();
+
 	/* The write_thread() callback is mandatory. */
 	if (WARN_ON(!con->write_thread))
 		return false;
@@ -1701,12 +1704,15 @@ bool nbcon_alloc(struct console *con)
 			return false;
 		}
 
-		if (printk_kthreads_running) {
+		if (printk_kthreads_ready && !have_boot_console) {
 			if (!nbcon_kthread_create(con)) {
 				kfree(con->pbufs);
 				con->pbufs = NULL;
 				return false;
 			}
+
+			/* Might be the first kthread. */
+			printk_kthreads_running = true;
 		}
 	}
 
@@ -1716,14 +1722,30 @@ bool nbcon_alloc(struct console *con)
 /**
  * nbcon_free - Free and cleanup the nbcon console specific data
  * @con:	Console to free/cleanup nbcon data
+ *
+ * Important: @have_nbcon_console must be updated before calling
+ *	this function. In particular, it can be set only when there
+ *	is still another nbcon console registered.
  */
 void nbcon_free(struct console *con)
 {
 	struct nbcon_state state = { };
 
-	if (printk_kthreads_running)
+	/* Synchronize the kthread stop. */
+	lockdep_assert_console_list_lock_held();
+
+	if (printk_kthreads_running) {
 		nbcon_kthread_stop(con);
 
+		/* Might be the last nbcon console.
+		 *
+		 * Do not rely on printk_kthreads_check_locked(). It is not
+		 * called in some code paths, see nbcon_free() callers.
+		 */
+		if (!have_nbcon_console)
+			printk_kthreads_running = false;
+	}
+
 	nbcon_state_set(con, &state);
 
 	/* Boot consoles share global printk buffers. */