Understanding panic and exit in the kernel

[mdf at FreeBSD.org]

On Wed, Sep 7, 2011 at 11:08 AM, Charlie Martin <crmartin at sgi.com> wrote:
> I'm still pursuing a FreeBSD bug in "7.2-PRERELEASE FreeBSD" -- and yes, we
> know this is wildly out of date, but it's not feasible to upgrade right now
> -- and while trying to backport a fix suggested here
> http://permalink.gmane.org/gmane.os.freebsd.current/134266 I got a situation
> where the panic calls in one of  these two macros from sys/queue.h
>
> #define    QMD_LIST_CHECK_NEXT(elm, field) do {                \
>    if (LIST_NEXT((elm), field) != NULL &&                \
>        LIST_NEXT((elm), field)->field.le_prev !=            \
> &((elm)->field.le_next))                    \
>             panic("Bad link elm %p next->prev != elm", (elm));    \
> } while (0)
>
> #define    QMD_LIST_CHECK_PREV(elm, field) do {                \
>    if (*(elm)->field.le_prev != (elm))                \
>        panic("Bad link elm %p prev->next != elm", (elm));    \
> } while (0)
>
> print the message, but *don't* then proceed to drop to the debugger --
> instead the system hangs, with the CPU running but I had no luck getting its
> attention to force it to the debugger.
>
> I'm not clear just what could be causing the hangup.
>
> For my immediate purposes, I'd be happy with any way in which I could
> brutally kill the kernel and force it to the debugger, say by replacing the
> panic call with a printf followed by "1/0;".  But I'm a little confused by
> the panic.c code -- it prints the arguments using a var_args, and then calls
> "exit(1);'

What file are you looking in?  The kernel panic() is in
sys/kern/kern_shutdown.c, not sys/boot/common/panic.c.  It will
optionally call kdb_enter_why() and then boot().

Do you have the debug.debugger_on_panic sysctl set to 1?

Thanks,
matthew

> So my questions:
>
> (1) will my brutal method actually force what I want or am I
> misunderstanding something
> (2) *which* of the several implementations of "int exit(int)" or similar is
> the one called in the FreeBSD kernel?
> (3) and how then does exit work?