From: John Ogness <john.ogness@linutronix.de>
Date: Tue, 26 Sep 2023 12:44:07 +0000
Subject: [PATCH 113/134] printk: Avoid console_lock dance if no legacy or boot
 consoles
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.6/older/patches-6.6.7-rt18.tar.xz

Currently the console lock is used to attempt legacy-type
printing even if there are no legacy or boot consoles registered.
If no such consoles are registered, the console lock does not
need to be taken.

Also, if boot consoles are registered, nbcon consoles must
perform their atomic printing under the console lock in order
to be synchronized with boot consoles.

Add tracking of legacy console registration and use it with
boot console tracking to avoid unnecessary code paths, i.e.
do not use the console lock if there are no boot consoles
and no legacy consoles.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/printk/internal.h |   12 +++++++++
 kernel/printk/printk.c   |   59 +++++++++++++++++++++++++++++++++++------------
 2 files changed, 56 insertions(+), 15 deletions(-)

--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -44,6 +44,16 @@ enum printk_info_flags {
 };
 
 extern struct printk_ringbuffer *prb;
+extern bool have_legacy_console;
+extern bool have_boot_console;
+
+/*
+ * Specifies if the console lock/unlock dance is needed for console
+ * printing. If @have_boot_console is true, the nbcon consoles will
+ * be printed serially along with the legacy consoles because nbcon
+ * consoles cannot print simultaneously with boot consoles.
+ */
+#define printing_via_unlock (have_legacy_console || have_boot_console)
 
 __printf(4, 0)
 int vprintk_store(int facility, int level,
@@ -122,6 +132,8 @@ static inline bool console_is_usable(str
 #define PRINTK_MESSAGE_MAX	0
 #define PRINTKRB_RECORD_MAX	0
 
+#define printing_via_unlock (false)
+
 /*
  * In !PRINTK builds we still export console_sem
  * semaphore and some of console functions (console_unlock()/etc.), so
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -464,6 +464,13 @@ static int console_msg_format = MSG_FORM
 static DEFINE_MUTEX(syslog_lock);
 
 /*
+ * Specifies if a legacy console is registered. If legacy consoles are
+ * present, it is necessary to perform the console_lock/console_unlock dance
+ * whenever console flushing should occur.
+ */
+bool have_legacy_console;
+
+/*
  * Specifies if a boot console is registered. If boot consoles are present,
  * nbcon consoles cannot print simultaneously and must be synchronized by
  * the console lock. This is because boot consoles and nbcon consoles may
@@ -2345,7 +2352,7 @@ asmlinkage int vprintk_emit(int facility
 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
 
 	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched) {
+	if (!in_sched && printing_via_unlock) {
 		/*
 		 * The caller may be holding system-critical or
 		 * timing-sensitive locks. Disable preemption during
@@ -2646,7 +2653,7 @@ void resume_console(void)
  */
 static int console_cpu_notify(unsigned int cpu)
 {
-	if (!cpuhp_tasks_frozen) {
+	if (!cpuhp_tasks_frozen && printing_via_unlock) {
 		/* If trylock fails, someone else is doing the printing */
 		if (console_trylock())
 			console_unlock();
@@ -3189,7 +3196,8 @@ void console_flush_on_panic(enum con_flu
 
 	nbcon_atomic_flush_all();
 
-	console_flush_all(false, &next_seq, &handover);
+	if (printing_via_unlock)
+		console_flush_all(false, &next_seq, &handover);
 }
 
 /*
@@ -3514,8 +3522,11 @@ void register_console(struct console *ne
 	newcon->dropped = 0;
 	console_init_seq(newcon, bootcon_registered);
 
-	if (newcon->flags & CON_NBCON)
+	if (newcon->flags & CON_NBCON) {
 		nbcon_init(newcon);
+	} else {
+		have_legacy_console = true;
+	}
 
 	if (newcon->flags & CON_BOOT)
 		have_boot_console = true;
@@ -3572,6 +3583,7 @@ EXPORT_SYMBOL(register_console);
 /* Must be called under console_list_lock(). */
 static int unregister_console_locked(struct console *console)
 {
+	bool found_legacy_con = false;
 	bool found_boot_con = false;
 	struct console *c;
 	int res;
@@ -3628,9 +3640,13 @@ static int unregister_console_locked(str
 	for_each_console(c) {
 		if (c->flags & CON_BOOT)
 			found_boot_con = true;
+		if (!(c->flags & CON_NBCON))
+			found_legacy_con = true;
 	}
 	if (!found_boot_con)
 		have_boot_console = false;
+	if (!found_legacy_con)
+		have_legacy_console = false;
 
 	return res;
 }
@@ -3782,6 +3798,7 @@ static bool __pr_flush(struct console *c
 	u64 last_diff = 0;
 	u64 printk_seq;
 	short flags;
+	bool locked;
 	int cookie;
 	u64 diff;
 	u64 seq;
@@ -3791,22 +3808,28 @@ static bool __pr_flush(struct console *c
 	seq = prb_next_reserve_seq(prb);
 
 	/* Flush the consoles so that records up to @seq are printed. */
-	console_lock();
-	console_unlock();
+	if (printing_via_unlock) {
+		console_lock();
+		console_unlock();
+	}
 
 	for (;;) {
 		unsigned long begin_jiffies;
 		unsigned long slept_jiffies;
 
+		locked = false;
 		diff = 0;
 
-		/*
-		 * Hold the console_lock to guarantee safe access to
-		 * console->seq. Releasing console_lock flushes more
-		 * records in case @seq is still not printed on all
-		 * usable consoles.
-		 */
-		console_lock();
+		if (printing_via_unlock) {
+			/*
+			 * Hold the console_lock to guarantee safe access to
+			 * console->seq. Releasing console_lock flushes more
+			 * records in case @seq is still not printed on all
+			 * usable consoles.
+			 */
+			console_lock();
+			locked = true;
+		}
 
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(c) {
@@ -3826,6 +3849,7 @@ static bool __pr_flush(struct console *c
 			if (flags & CON_NBCON) {
 				printk_seq = nbcon_seq_read(c);
 			} else {
+				WARN_ON_ONCE(!locked);
 				printk_seq = c->seq;
 			}
 
@@ -3837,7 +3861,8 @@ static bool __pr_flush(struct console *c
 		if (diff != last_diff && reset_on_progress)
 			remaining_jiffies = timeout_jiffies;
 
-		console_unlock();
+		if (locked)
+			console_unlock();
 
 		/* Note: @diff is 0 if there are no usable consoles. */
 		if (diff == 0 || remaining_jiffies == 0)
@@ -3959,7 +3984,11 @@ void defer_console_output(void)
 	 * New messages may have been added directly to the ringbuffer
 	 * using vprintk_store(), so wake any waiters as well.
 	 */
-	__wake_up_klogd(PRINTK_PENDING_WAKEUP | PRINTK_PENDING_OUTPUT);
+	int val = PRINTK_PENDING_WAKEUP;
+
+	if (printing_via_unlock)
+		val |= PRINTK_PENDING_OUTPUT;
+	__wake_up_klogd(val);
 }
 
 void printk_trigger_flush(void)
