printk: Consolidate console deferred printing

Printing to consoles can be deferred for several reasons:

- explicitly with printk_deferred()
- printk() in NMI context
- recursive printk() calls

The current implementation is not consistent. For printk_deferred(),
irq work is scheduled twice. For NMI und recursive, panic CPU
suppression and caller delays are not properly enforced.

Correct these inconsistencies by consolidating the deferred printing
code so that vprintk_deferred() is the top-level function for
deferred printing and vprintk_emit() will perform whichever irq_work
queueing is appropriate.

Also add kerneldoc for wake_up_klogd() and defer_console_output() to
clarify their differences and appropriate usage.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230717194607.145135-6-john.ogness@linutronix.de

kernel/printk/printk.c

@@ -2306,7 +2306,11 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
-	wake_up_klogd();
+	if (in_sched)
+		defer_console_output();
+	else
+		wake_up_klogd();
+
 	return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);
@@ -3841,11 +3845,33 @@ static void __wake_up_klogd(int val)
 	preempt_enable();
 }
 
+/**
+ * wake_up_klogd - Wake kernel logging daemon
+ *
+ * Use this function when new records have been added to the ringbuffer
+ * and the console printing of those records has already occurred or is
+ * known to be handled by some other context. This function will only
+ * wake the logging daemon.
+ *
+ * Context: Any context.
+ */
 void wake_up_klogd(void)
 {
 	__wake_up_klogd(PRINTK_PENDING_WAKEUP);
 }
 
+/**
+ * defer_console_output - Wake kernel logging daemon and trigger
+ *	console printing in a deferred context
+ *
+ * Use this function when new records have been added to the ringbuffer,
+ * this context is responsible for console printing those records, but
+ * the current context is not allowed to perform the console printing.
+ * Trigger an irq_work context to perform the console printing. This
+ * function also wakes the logging daemon.
+ *
+ * Context: Any context.
+ */
 void defer_console_output(void)
 {
 	/*
@@ -3862,12 +3888,7 @@ void printk_trigger_flush(void)
 
 int vprintk_deferred(const char *fmt, va_list args)
 {
-	int r;
-
-	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
-	defer_console_output();
-
-	return r;
+	return vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
 }
 
 int _printk_deferred(const char *fmt, ...)

kernel/printk/printk_safe.c

@@ -38,13 +38,8 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	 * Use the main logbuf even in NMI. But avoid calling console
 	 * drivers that might have their own locks.
 	 */
-	if (this_cpu_read(printk_context) || in_nmi()) {
-		int len;
-
-		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
-		defer_console_output();
-		return len;
-	}
+	if (this_cpu_read(printk_context) || in_nmi())
+		return vprintk_deferred(fmt, args);
 
 	/* No obstacles. */
 	return vprintk_default(fmt, args);