[debug patch] printk: Add a printk killswitch to robustify NMIwatchdog messages

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxx> wrote:

> If the ticks stop this suggests a lockup within the printk code.
> [...]

In which case the printk-killswitch patch below (to be applied 
*instead* of the previous debugging patch i sent) should provide the 
desired NMI watchdog output on the serial console.

Warning: it's entirely untested.

Thanks,

	Ingo

 arch/x86/kernel/early_printk.c |    2 +-
 include/linux/printk.h         |    4 ++++
 kernel/printk.c                |   18 ++++++++++++++++++
 kernel/watchdog.c              |    7 +++++++
 4 files changed, 30 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index cd28a35..d75fd66 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -171,7 +171,7 @@ static struct console early_serial_console = {
 
 /* Direct interface for emergencies */
 static struct console *early_console = &early_vga_console;
-static int __initdata early_console_initialized;
+int early_console_initialized;
 
 asmlinkage void early_printk(const char *fmt, ...)
 {
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 0101d55..7393291 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -88,6 +88,8 @@ int no_printk(const char *fmt, ...)
 	return 0;
 }
 
+extern int early_console_initialized;
+
 extern asmlinkage __attribute__ ((format (printf, 1, 2)))
 void early_printk(const char *fmt, ...);
 
@@ -114,6 +116,8 @@ extern int printk_delay_msec;
 extern int dmesg_restrict;
 extern int kptr_restrict;
 
+extern void printk_kill(void);
+
 void log_buf_kexec_setup(void);
 void __init setup_log_buf(int early);
 #else
diff --git a/kernel/printk.c b/kernel/printk.c
index 3518539..f6193e1 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -519,6 +519,19 @@ static void __call_console_drivers(unsigned start, unsigned end)
 	}
 }
 
+/*
+ * 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 int __read_mostly printk_killswitch;
+
+void printk_kill(void)
+{
+	printk_killswitch = 1;
+}
+
 static int __read_mostly ignore_loglevel;
 
 static int __init ignore_loglevel_setup(char *str)
@@ -833,6 +846,10 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	size_t plen;
 	char special;
 
+	/* Return early if a debugging subsystem has killed printk output: */
+	if (unlikely(printk_killswitch))
+		return 1;
+
 	boot_delay_msec();
 	printk_delay();
 
@@ -1533,6 +1550,7 @@ void register_console(struct console *newcon)
 		for_each_console(bcon)
 			if (bcon->flags & CON_BOOT)
 				unregister_console(bcon);
+		early_console_initialized = 0;
 	} else {
 		printk(KERN_INFO "%sconsole [%s%d] enabled\n",
 			(newcon->flags & CON_BOOT) ? "boot" : "" ,
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 3d0c56a..6e9b109 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -234,6 +234,13 @@ static void watchdog_overflow_callback(struct perf_event *event, int nmi,
 		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:
+		 */
+		if (early_console_initialized)
+			printk_kill();
+
 		if (hardlockup_panic)
 			panic("Watchdog detected hard LOCKUP on cpu %d", this_cpu);
 		else
--
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