[PATCH printk v2 09/18] printk: nbcon: Start printing threads

[John Ogness]

If there are no boot consoles, the printing threads are started
in early_initcall.

If there are boot consoles, the printing threads are started
after the last boot console has unregistered. The printing
threads do not need to be concerned about boot consoles because
boot consoles cannot register once a non-boot console has
registered.

Until a printing thread of a console has started, that console
will print using atomic_write() in the printk() caller context.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 kernel/printk/internal.h |  2 ++
 kernel/printk/nbcon.c    | 18 +++++++++++++++++-
 kernel/printk/printk.c   | 14 ++++++++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index b2b5c10a4321..5d4722310850 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -48,6 +48,7 @@ struct printk_ringbuffer;
 struct dev_printk_info;
 
 extern struct printk_ringbuffer *prb;
+extern bool printk_threads_enabled;
 
 __printf(4, 0)
 int vprintk_store(int facility, int level,
@@ -161,6 +162,7 @@ static inline void nbcon_kthread_wake(struct console *con)
 
 static inline void nbcon_kthread_wake(struct console *con) { }
 static inline void nbcon_kthread_create(struct console *con) { }
+#define printk_threads_enabled (false)
 
 /*
  * In !PRINTK builds we still export console_sem
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index f60d47b5db8a..480a0ced2708 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -213,6 +213,8 @@ static void nbcon_seq_try_update(struct nbcon_context *ctxt, u64 new_seq)
 	}
 }
 
+bool printk_threads_enabled __ro_after_init;
+
 /**
  * nbcon_context_try_acquire_direct - Try to acquire directly
  * @ctxt:	The context of the caller
@@ -1542,7 +1544,7 @@ void nbcon_kthread_create(struct console *con)
 	if (!(con->flags & CON_NBCON) || !con->write_thread)
 		return;
 
-	if (con->kthread)
+	if (!printk_threads_enabled || con->kthread)
 		return;
 
 	/*
@@ -1568,6 +1570,19 @@ void nbcon_kthread_create(struct console *con)
 	sched_set_normal(con->kthread, -20);
 }
 
+static int __init printk_setup_threads(void)
+{
+	struct console *con;
+
+	console_list_lock();
+	printk_threads_enabled = true;
+	for_each_console(con)
+		nbcon_kthread_create(con);
+	console_list_unlock();
+	return 0;
+}
+early_initcall(printk_setup_threads);
+
 /**
  * nbcon_alloc - Allocate buffers needed by the nbcon console
  * @con:	Console to allocate buffers for
@@ -1617,6 +1632,7 @@ void nbcon_init(struct console *con, u64 init_seq)
 	init_irq_work(&con->irq_work, nbcon_irq_work);
 	nbcon_seq_force(con, init_seq);
 	nbcon_state_set(con, &state);
+	nbcon_kthread_create(con);
 }
 
 /**
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 457c01311a95..4613f9145a8e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2389,6 +2389,9 @@ asmlinkage int vprintk_emit(int facility, int level,
 		 *
 		 * - When this CPU is in panic.
 		 *
+		 * - When booting, before the printing threads have been
+		 *   started.
+		 *
 		 * - During shutdown, since the printing threads may not get
 		 *   a chance to print the final messages.
 		 *
@@ -2397,6 +2400,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 		 * consoles cannot print simultaneously with boot consoles.
 		 */
 		if (is_panic_context ||
+		    !printk_threads_enabled ||
 		    (system_state > SYSTEM_RUNNING)) {
 			nbcon_atomic_flush_pending();
 		}
@@ -3742,6 +3746,7 @@ EXPORT_SYMBOL(register_console);
 static int unregister_console_locked(struct console *console)
 {
 	bool use_device_lock = (console->flags & CON_NBCON) && console->write_atomic;
+	bool is_boot_con = (console->flags & CON_BOOT);
 	bool found_legacy_con = false;
 	bool found_nbcon_con = false;
 	bool found_boot_con = false;
@@ -3824,6 +3829,15 @@ static int unregister_console_locked(struct console *console)
 	if (!found_nbcon_con)
 		have_nbcon_console = found_nbcon_con;
 
+	/*
+	 * When the last boot console unregisters, start up the
+	 * printing threads.
+	 */
+	if (is_boot_con && !have_boot_console) {
+		for_each_console(c)
+			nbcon_kthread_create(c);
+	}
+
 	return res;
 }
 
-- 
2.39.2