Re: [PATCH] panic: fix incomplete panic log in panic()

Andrew Morton
2012-10-11T21:18:48Z

On Thu, 11 Oct 2012 16:03:07 +0800
Qing Zhu <qzhu@marvell.com> wrote:

> Panic log should be printed on the console, but if someone lock the
> console when panic, console won't print out panic log.
>
> The incomplete panic log issue will happen in below scenarios:
> 1. One task call console_lock(), then panic happend before it call
> console_unlock(). No panic log can be printed.
> 2. Cpu 0 call panic()->Cpu 1 call console_lock()->Cpu 0 call
> smp_send_stop()
> Cpu 1 will be stopped and can't unlock console,only top part of panic log
> will be printed.
>
> So unlock console anyway in panic() to keep panic log printed.
>
> Signed-off-by: Qing Zhu <qzhu@marvell.com>
> ---
> kernel/panic.c | 8 ++++++++
> 1 files changed, 8 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/panic.c b/kernel/panic.c
> index e1b2822..3924d25 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -23,6 +23,7 @@
> #include <linux/init.h>
> #include <linux/nmi.h>
> #include <linux/dmi.h>
> +#include <linux/console.h>
>
> #define PANIC_TIMER_STEP 100
> #define PANIC_BLINK_SPD 18
> @@ -127,6 +128,13 @@ void panic(const char *fmt, ...)
>
> atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
>
> + /*
> + * Unlock the console anyway here, in case it's occupied by another
> + * one which has no chance to unlock the console thus prevents the
> + * panic log prints on the console.
> + */
> + console_unlock();
> +
> bust_spinlocks(0);
>
> if (!panic_blink)

hm. console_unlock() does a large amount of work, and it seems risky
to do all of that when the system is in a bad-and-getting-worse state.

Is there some more modest thing we could do here, for example,

if (console_locked) {
up(&console_sem);
console_locked = 0;
}

or something along those lines?

Also, perhaps this operation should be moved into bust_spinlocks().
What would have happened if your code had triggered an oops, rather
than called panic()?