From: Ingo Molnar <mingo@elte.hu>
Date: Fri, 22 Jul 2011 17:58:40 +0200
Subject: printk: Add a printk kill switch
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 3d54f787529f3cfe58fc3d80f094cb10dd54574e
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

Add a prinkt-kill-switch. This is used from (NMI) watchdog to ensure that
it does not dead-lock with the early printk code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 include/linux/printk.h |    2 +
 kernel/printk/printk.c |   79 ++++++++++++++++++++++++++++++++++++-------------
 kernel/watchdog_hld.c  |    9 +++++
 3 files changed, 70 insertions(+), 20 deletions(-)

--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -141,9 +141,11 @@ struct va_format {
 #ifdef CONFIG_EARLY_PRINTK
 extern asmlinkage __printf(1, 2)
 void early_printk(const char *fmt, ...);
+extern void printk_kill(void);
 #else
 static inline __printf(1, 2) __cold
 void early_printk(const char *s, ...) { }
+static inline void printk_kill(void) { }
 #endif
 
 #ifdef CONFIG_PRINTK_NMI
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -400,6 +400,58 @@ DEFINE_RAW_SPINLOCK(logbuf_lock);
 		printk_safe_exit_irqrestore(flags);	\
 	} while (0)
 
+#ifdef CONFIG_EARLY_PRINTK
+struct console *early_console;
+
+static void early_vprintk(const char *fmt, va_list ap)
+{
+	if (early_console) {
+		char buf[512];
+		int n = vscnprintf(buf, sizeof(buf), fmt, ap);
+
+		early_console->write(early_console, buf, n);
+	}
+}
+
+asmlinkage void early_printk(const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	early_vprintk(fmt, ap);
+	va_end(ap);
+}
+
+/*
+ * This is independent of any log levels - a global
+ * kill switch that turns off all of printk.
+ *
+ * Used by the NMI watchdog if early-printk is enabled.
+ */
+static bool __read_mostly printk_killswitch;
+
+void printk_kill(void)
+{
+	printk_killswitch = true;
+}
+
+#ifdef CONFIG_PRINTK
+static int forced_early_printk(const char *fmt, va_list ap)
+{
+	if (!printk_killswitch)
+		return 0;
+	early_vprintk(fmt, ap);
+	return 1;
+}
+#endif
+
+#else
+static inline int forced_early_printk(const char *fmt, va_list ap)
+{
+	return 0;
+}
+#endif
+
 #ifdef CONFIG_PRINTK
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
 /* the next printk record to read by syslog(READ) or /proc/kmsg */
@@ -1844,6 +1896,13 @@ asmlinkage int vprintk_emit(int facility
 	int printed_len = 0;
 	bool in_sched = false;
 
+	/*
+	 * Fall back to early_printk if a debugging subsystem has
+	 * killed printk output
+	 */
+	if (unlikely(forced_early_printk(fmt, args)))
+		return 1;
+
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
 		in_sched = true;
@@ -2024,26 +2083,6 @@ static bool suppress_message_printing(in
 
 #endif /* CONFIG_PRINTK */
 
-#ifdef CONFIG_EARLY_PRINTK
-struct console *early_console;
-
-asmlinkage __visible void early_printk(const char *fmt, ...)
-{
-	va_list ap;
-	char buf[512];
-	int n;
-
-	if (!early_console)
-		return;
-
-	va_start(ap, fmt);
-	n = vscnprintf(buf, sizeof(buf), fmt, ap);
-	va_end(ap);
-
-	early_console->write(early_console, buf, n);
-}
-#endif
-
 static int __add_preferred_console(char *name, int idx, char *options,
 				   char *brl_options)
 {
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -21,6 +21,7 @@
 static DEFINE_PER_CPU(bool, hard_watchdog_warn);
 static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
 static DEFINE_PER_CPU(struct perf_event *, watchdog_ev);
+static DEFINE_RAW_SPINLOCK(watchdog_output_lock);
 
 /* boot commands */
 /*
@@ -165,6 +166,13 @@ static void watchdog_overflow_callback(s
 		/* only print hardlockups once */
 		if (__this_cpu_read(hard_watchdog_warn) == true)
 			return;
+		/*
+		 * If early-printk is enabled then make sure we do not
+		 * lock up in printk() and kill console logging:
+		 */
+		printk_kill();
+
+		raw_spin_lock(&watchdog_output_lock);
 
 		pr_emerg("Watchdog detected hard LOCKUP on cpu %d", this_cpu);
 		print_modules();
@@ -182,6 +190,7 @@ static void watchdog_overflow_callback(s
 				!test_and_set_bit(0, &hardlockup_allcpu_dumped))
 			trigger_allbutself_cpu_backtrace();
 
+		raw_spin_unlock(&watchdog_output_lock);
 		if (hardlockup_panic)
 			nmi_panic(regs, "Hard LOCKUP");