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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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