kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers

Add a "crash_kexec_post_notifiers" boot option to run kdump after
running panic_notifiers and dump kmsg.  This can help rare situations
where kdump fails because of unstable crashed kernel or hardware failure
(memory corruption on critical data/code), or the 2nd kernel is already
broken by the 1st kernel (it's a broken behavior, but who can guarantee
that the "crashed" kernel works correctly?).

Usage: add "crash_kexec_post_notifiers" to kernel boot option.

Note that this actually increases risks of the failure of kdump.  This
option should be set only if you worry about the rare case of kdump
failure rather than increasing the chance of success.
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Motohiro Kosaki <Motohiro.Kosaki@us.fujitsu.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
Cc: Satoru MORIYA <satoru.moriya.br@hitachi.com>
Cc: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -2361,6 +2361,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			timeout < 0: reboot immediately
 			Format: <timeout>
 
+	crash_kexec_post_notifiers
+			Run kdump after running panic-notifiers and dumping
+			kmsg. This only for the users who doubt kdump always
+			succeeds in any situation.
+			Note that this also increases risks of kdump failure,
+			because some panic notifiers can make the crashed
+			kernel more unstable.
+
 	parkbd.port=	[HW] Parallel port number the keyboard adapter is
 			connected to, default is 0.
 			Format: <parport#>

kernel/panic.c

@@ -32,6 +32,7 @@ static unsigned long tainted_mask;
 static int pause_on_oops;
 static int pause_on_oops_flag;
 static DEFINE_SPINLOCK(pause_on_oops_lock);
+static bool crash_kexec_post_notifiers;
 
 int panic_timeout = CONFIG_PANIC_TIMEOUT;
 EXPORT_SYMBOL_GPL(panic_timeout);
@@ -112,8 +113,10 @@ void panic(const char *fmt, ...)
 	/*
 	 * If we have crashed and we have a crash kernel loaded let it handle
 	 * everything else.
-	 * Do we want to call this before we try to display a message?
+	 * If we want to run this after calling panic_notifiers, pass
+	 * the "crash_kexec_post_notifiers" option to the kernel.
 	 */
+	if (!crash_kexec_post_notifiers)
 		crash_kexec(NULL);
 
 	/*
@@ -131,6 +134,15 @@ void panic(const char *fmt, ...)
 
 	kmsg_dump(KMSG_DUMP_PANIC);
 
+	/*
+	 * If you doubt kdump always works fine in any situation,
+	 * "crash_kexec_post_notifiers" offers you a chance to run
+	 * panic_notifiers and dumping kmsg before kdump.
+	 * Note: since some panic_notifiers can make crashed kernel
+	 * more unstable, it can increase risks of the kdump failure too.
+	 */
+	crash_kexec(NULL);
+
 	bust_spinlocks(0);
 
 	if (!panic_blink)
@@ -472,6 +484,13 @@ EXPORT_SYMBOL(__stack_chk_fail);
 core_param(panic, panic_timeout, int, 0644);
 core_param(pause_on_oops, pause_on_oops, int, 0644);
 
+static int __init setup_crash_kexec_post_notifiers(char *s)
+{
+	crash_kexec_post_notifiers = true;
+	return 0;
+}
+early_param("crash_kexec_post_notifiers", setup_crash_kexec_post_notifiers);
+
 static int __init oops_setup(char *s)
 {
 	if (!s)