printk: add pr_flush()

Provide a might-sleep function to allow waiting for console printers
to catch up to the latest logged message.

Use pr_flush() whenever it is desirable to get buffered messages
printed before continuing: suspend_console(), resume_console(),
console_stop(), console_start(), console_unblank().
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220421212250.565456-12-john.ogness@linutronix.de

include/linux/printk.h

@@ -170,6 +170,8 @@ extern void __printk_safe_exit(void);
 #define printk_deferred_enter __printk_safe_enter
 #define printk_deferred_exit __printk_safe_exit
 
+extern bool pr_flush(int timeout_ms, bool reset_on_progress);
+
 /*
  * Please don't use printk_ratelimit(), because it shares ratelimiting state
  * with all other unrelated printk_ratelimit() callsites.  Instead use
@@ -220,6 +222,11 @@ static inline void printk_deferred_exit(void)
 {
 }
 
+static inline bool pr_flush(int timeout_ms, bool reset_on_progress)
+{
+	return true;
+}
+
 static inline int printk_ratelimit(void)
 {
 	return 0;

kernel/printk/printk.c

@@ -2296,6 +2296,8 @@ asmlinkage __visible int _printk(const char *fmt, ...)
 }
 EXPORT_SYMBOL(_printk);
 
+static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress);
+
 #else /* CONFIG_PRINTK */
 
 #define CONSOLE_LOG_MAX		0
@@ -2328,6 +2330,7 @@ static void call_console_driver(struct console *con, const char *text, size_t le
 {
 }
 static bool suppress_message_printing(int level) { return false; }
+static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) { return true; }
 
 #endif /* CONFIG_PRINTK */
 
@@ -2515,6 +2518,7 @@ void suspend_console(void)
 	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();
@@ -2527,6 +2531,7 @@ void resume_console(void)
 	down_console_sem();
 	console_suspended = 0;
 	console_unlock();
+	pr_flush(1000, true);
 }
 
 /**
@@ -2912,6 +2917,9 @@ void console_unblank(void)
 		if ((c->flags & CON_ENABLED) && c->unblank)
 			c->unblank();
 	console_unlock();
+
+	if (!oops_in_progress)
+		pr_flush(1000, true);
 }
 
 /**
@@ -2970,6 +2978,7 @@ struct tty_driver *console_device(int *index)
  */
 void console_stop(struct console *console)
 {
+	__pr_flush(console, 1000, true);
 	console_lock();
 	console->flags &= ~CON_ENABLED;
 	console_unlock();
@@ -2981,6 +2990,7 @@ void console_start(struct console *console)
 	console_lock();
 	console->flags |= CON_ENABLED;
 	console_unlock();
+	__pr_flush(console, 1000, true);
 }
 EXPORT_SYMBOL(console_start);
 
@@ -3352,6 +3362,79 @@ static int __init printk_late_init(void)
 late_initcall(printk_late_init);
 
 #if defined CONFIG_PRINTK
+/* If @con is specified, only wait for that console. Otherwise wait for all. */
+static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress)
+{
+	int remaining = timeout_ms;
+	struct console *c;
+	u64 last_diff = 0;
+	u64 printk_seq;
+	u64 diff;
+	u64 seq;
+
+	might_sleep();
+
+	seq = prb_next_seq(prb);
+
+	for (;;) {
+		diff = 0;
+
+		console_lock();
+		for_each_console(c) {
+			if (con && con != c)
+				continue;
+			if (!console_is_usable(c))
+				continue;
+			printk_seq = c->seq;
+			if (printk_seq < seq)
+				diff += seq - printk_seq;
+		}
+		console_unlock();
+
+		if (diff != last_diff && reset_on_progress)
+			remaining = timeout_ms;
+
+		if (diff == 0 || remaining == 0)
+			break;
+
+		if (remaining < 0) {
+			/* no timeout limit */
+			msleep(100);
+		} else if (remaining < 100) {
+			msleep(remaining);
+			remaining = 0;
+		} else {
+			msleep(100);
+			remaining -= 100;
+		}
+
+		last_diff = diff;
+	}
+
+	return (diff == 0);
+}
+
+/**
+ * pr_flush() - Wait for printing threads to catch up.
+ *
+ * @timeout_ms:        The maximum time (in ms) to wait.
+ * @reset_on_progress: Reset the timeout if forward progress is seen.
+ *
+ * A value of 0 for @timeout_ms means no waiting will occur. A value of -1
+ * represents infinite waiting.
+ *
+ * If @reset_on_progress is true, the timeout will be reset whenever any
+ * 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.
+ */
+bool pr_flush(int timeout_ms, bool reset_on_progress)
+{
+	return __pr_flush(NULL, timeout_ms, reset_on_progress);
+}
+EXPORT_SYMBOL(pr_flush);
+
 /*
  * Delayed printk version, for scheduler-internal messages:
  */