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Subject: printk: Add a printk kill switch
From: Ingo Molnar <mingo@elte.hu>
Date: Fri, 22 Jul 2011 17:58:40 +0200
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>
---
include/linux/printk.h | 2 +
kernel/printk/printk.c | 79 ++++++++++++++++++++++++++++++++++++-------------
kernel/watchdog_hld.c | 10 ++++++
3 files changed, 71 insertions(+), 20 deletions(-)
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -140,9 +140,11 @@ 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
@@ -404,6 +404,58 @@ 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 */
@@ -1886,6 +1938,13 @@ asmlinkage int vprintk_emit(int facility
bool in_sched = false;
unsigned long flags;
+ /*
+ * 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;
@@ -2026,26 +2085,6 @@ 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
@@ -24,6 +24,8 @@ static DEFINE_PER_CPU(bool, hard_watchdo
static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
static DEFINE_PER_CPU(struct perf_event *, watchdog_ev);
static DEFINE_PER_CPU(struct perf_event *, dead_event);
+static DEFINE_RAW_SPINLOCK(watchdog_output_lock);
+
static struct cpumask dead_events_mask;
static unsigned long hardlockup_allcpu_dumped;
@@ -134,6 +136,13 @@ 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();
@@ -151,6 +160,7 @@ 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");
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