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SubjectRe: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages
On 05.06.2011 16:06, Ingo Molnar wrote:
>
> * Arne Jansen<lists@die-jansens.de> wrote:
>
>> On 05.06.2011 15:39, Ingo Molnar wrote:
>>>
>>> * Arne Jansen<lists@die-jansens.de> wrote:
>>>
>>>>>> Warning: it's entirely untested.
>>>>
>>>> How is the output supposed to come through? shouldn't printk revert
>>>> to early_printk instead of just returning?
>>>
>>> oh, right you are.
>>>
>>> Does the patch below work? It does early-printk within printk().
>>
>> Too late, I already built my on early_vprintk ;)
>
> heh :-)
>
> Mind posting the patch? Your tested patch is infinitely more valuable
> than my not-even-build-tested patch ;-)

It's basically just your patch. I attached it nevertheless...

-Arne

diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index cd28a35..0623126 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, ...)
{
@@ -185,6 +185,15 @@ asmlinkage void early_printk(const char *fmt, ...)
va_end(ap);
}

+asmlinkage void early_vprintk(const char *fmt, va_list ap)
+{
+ char buf[512];
+ int n;
+
+ n = vscnprintf(buf, sizeof(buf), fmt, ap);
+ early_console->write(early_console, buf, n);
+}
+
static inline void early_console_register(struct console *con, int keep_early)
{
if (early_console->index != -1) {
diff --git a/include/linux/printk.h b/include/linux/printk.h
index ee048e7..6bb6963 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -86,8 +86,11 @@ 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, ...);
+void early_vprintk(const char *fmt, va_list args);

extern int printk_needs_cpu(int cpu);
extern void printk_tick(void);
@@ -112,6 +115,8 @@ extern int printk_delay_msec;
extern int dmesg_restrict;
extern int kptr_restrict;

+extern void printk_kill(void);
+
void log_buf_kexec_setup(void);
#else
static inline __attribute__ ((format (printf, 1, 0)))
diff --git a/kernel/printk.c b/kernel/printk.c
index da8ca81..38f880f 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -490,6 +490,19 @@ 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)
@@ -804,6 +817,15 @@ asmlinkage int vprintk(const char *fmt, va_list args)
size_t plen;
char special;

+ /*
+ * Fall back to early_printk if a debugging subsystem has
+ * killed printk output
+ */
+ if (unlikely(printk_killswitch)) {
+ early_vprintk(fmt, args);
+ return 1;
+ }
+
boot_delay_msec();
printk_delay();

@@ -1504,6 +1526,7 @@ 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 140dce7..18fca3d 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -197,6 +197,8 @@ static struct perf_event_attr wd_hw_attr = {
.disabled = 1,
};

+static DEFINE_SPINLOCK(watchdog_output_lock);
+
/* Callback function for perf event subsystem */
static void watchdog_overflow_callback(struct perf_event *event, int nmi,
struct perf_sample_data *data,
@@ -223,10 +225,20 @@ static void watchdog_overflow_callback(struct perf_event *event, int nmi,
if (__this_cpu_read(hard_watchdog_warn) == true)
return;

- if (hardlockup_panic)
+ /*
+ * 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
+ } else {
+ spin_lock(&watchdog_output_lock);
WARN(1, "Watchdog detected hard LOCKUP on cpu %d", this_cpu);
+ spin_unlock(&watchdog_output_lock);
+ }

__this_cpu_write(hard_watchdog_warn, true);
return;
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