printk: Add per-console suspended state

Currently the global @console_suspended is used to determine if
consoles are in a suspended state. Its primary purpose is to allow
usage of the console_lock when suspended without causing console
printing. It is synchronized by the console_lock.

Rather than relying on the console_lock to determine suspended
state, make it an official per-console state that is set within
console->flags. This allows the state to be queried via SRCU.

Remove @console_suspended. Console printing will still be avoided
when suspended because console_is_usable() returns false when
the new suspended flag is set for that console.

Signed-off-by: John Ogness <[e-mail]>
Reviewed-by: Sergey Senozhatsky <[e-mail]>
Reviewed-by: Petr Mladek <[e-mail]>
Signed-off-by: Petr Mladek <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/console.h

···
  *			receiving the printk spam for obvious reasons.
  * @CON_EXTENDED:	The console supports the extended output format of
  *			/dev/kmesg which requires a larger output buffer.
+ * @CON_SUSPENDED:	Indicates if a console is suspended. If true, the
+ *			printing callbacks must not be called.
  */
 enum cons_flags {
 	CON_PRINTBUFFER		= BIT(0),
···
 	CON_ANYTIME		= BIT(4),
 	CON_BRL			= BIT(5),
 	CON_EXTENDED		= BIT(6),
+	CON_SUSPENDED		= BIT(7),
 };
 
 /**

kernel/printk/printk.c

···
 static DEFINE_MUTEX(console_mutex);
 
 /*
- * console_sem protects updates to console->seq and console_suspended,
+ * console_sem protects updates to console->seq
  * and also provides serialization for console printing.
  */
 static DEFINE_SEMAPHORE(console_sem);
···
  * paths in the console code where we end up in places I want
  * locked without the console semaphore held).
  */
-static int console_locked, console_suspended;
+static int console_locked;
 
 /*
  *	Array of consoles built from command line options (console=)
···
  */
 void suspend_console(void)
 {
+	struct console *con;
+
 	if (!console_suspend_enabled)
 		return;
 	pr_info("Suspending console(s) (use no_console_suspend to debug)\n");
 	pr_flush(1000, true);
-	console_lock();
-	console_suspended = 1;
-	up_console_sem();
+
+	console_list_lock();
+	for_each_console(con)
+		console_srcu_write_flags(con, con->flags | CON_SUSPENDED);
+	console_list_unlock();
+
+	/*
+	 * Ensure that all SRCU list walks have completed. All printing
+	 * contexts must be able to see that they are suspended so that it
+	 * is guaranteed that all printing has stopped when this function
+	 * completes.
+	 */
+	synchronize_srcu(&console_srcu);
 }
 
 void resume_console(void)
 {
+	struct console *con;
+
 	if (!console_suspend_enabled)
 		return;
-	down_console_sem();
-	console_suspended = 0;
-	console_unlock();
+
+	console_list_lock();
+	for_each_console(con)
+		console_srcu_write_flags(con, con->flags & ~CON_SUSPENDED);
+	console_list_unlock();
+
+	/*
+	 * Ensure that all SRCU list walks have completed. All printing
+	 * contexts must be able to see they are no longer suspended so
+	 * that they are guaranteed to wake up and resume printing.
+	 */
+	synchronize_srcu(&console_srcu);
+
 	pr_flush(1000, true);
 }
 
···
 		msleep(1000);
 
 	down_console_sem();
-	if (console_suspended)
-		return;
 	console_locked = 1;
 	console_may_schedule = 1;
 }
···
 		return 0;
 	if (down_trylock_console_sem())
 		return 0;
-	if (console_suspended) {
-		up_console_sem();
-		return 0;
-	}
 	console_locked = 1;
 	console_may_schedule = 0;
 	return 1;
···
 	short flags = console_srcu_read_flags(con);
 
 	if (!(flags & CON_ENABLED))
+		return false;
+
+	if ((flags & CON_SUSPENDED))
 		return false;
 
 	if (!con->write)
···
 	bool handover;
 	bool flushed;
 	u64 next_seq;
-
-	if (console_suspended) {
-		up_console_sem();
-		return;
-	}
 
 	/*
 	 * Console drivers are called with interrupts disabled, so
···
 
 		/*
 		 * Hold the console_lock to guarantee safe access to
-		 * console->seq and to prevent changes to @console_suspended
-		 * until all consoles have been processed.
+		 * console->seq.
 		 */
 		console_lock();
 
···
 		for_each_console_srcu(c) {
 			if (con && con != c)
 				continue;
+			/*
+			 * If consoles are not usable, it cannot be expected
+			 * that they make forward progress, so only increment
+			 * @diff for usable consoles.
+			 */
 			if (!console_is_usable(c))
 				continue;
 			printk_seq = c->seq;
···
 		}
 		console_srcu_read_unlock(cookie);
 
-		/*
-		 * If consoles are suspended, it cannot be expected that they
-		 * make forward progress, so timeout immediately. @diff is
-		 * still used to return a valid flush status.
-		 */
-		if (console_suspended)
-			remaining = 0;
-		else if (diff != last_diff && reset_on_progress)
+		if (diff != last_diff && reset_on_progress)
 			remaining = timeout_ms;
 
 		console_unlock();
 
+		/* Note: @diff is 0 if there are no usable consoles. */
 		if (diff == 0 || remaining == 0)
 			break;
 
···
  * printer has been seen to make some forward progress.
  *
  * Context: Process context. May sleep while acquiring console lock.
- * Return: true if all enabled printers are caught up.
+ * Return: true if all usable printers are caught up.
  */
 static bool pr_flush(int timeout_ms, bool reset_on_progress)
 {