panic: panic=-1 for immediate reboot

When a kernel BUG or oops occurs, ChromeOS intends to panic and
immediately reboot, with stacktrace and other messages preserved in RAM
across reboot.

But the longer we delay, the more likely the user is to poweroff and
lose the info.

panic_timeout (seconds before rebooting) is set by panic= boot option or
sysctl or /proc/sys/kernel/panic; but 0 means wait forever, so at
present we have to delay at least 1 second.

Let a negative number mean reboot immediately (with the small cosmetic
benefit of suppressing that newline-less "Rebooting in %d seconds.."
message).

Signed-off-by: Hugh Dickins <hughd@chromium.org>
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Olaf Hering <olaf@aepfle.de>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -1846,7 +1846,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			See Documentation/sound/oss/oss-parameters.txt
 
 	panic=		[KNL] Kernel behaviour on panic: delay <timeout>
-			seconds before rebooting
+			timeout > 0: seconds before rebooting
+			timeout = 0: wait forever
+			timeout < 0: reboot immediately
 			Format: <timeout>
 
 	parkbd.port=	[HW] Parallel port number the keyboard adapter is

kernel/panic.c

@@ -119,6 +119,8 @@ NORET_TYPE void panic(const char * fmt, ...)
 			}
 			mdelay(PANIC_TIMER_STEP);
 		}
+	}
+	if (panic_timeout != 0) {
 		/*
 		 * This will not be a clean reboot, with everything
 		 * shutting down.  But if there is a chance of