kernel/panic/kexec: fix "crash_kexec_post_notifiers" option issue in oops path

Commit f06e5153f4ae2e ("kernel/panic.c: add "crash_kexec_post_notifiers"
option for kdump after panic_notifers") introduced
"crash_kexec_post_notifiers" kernel boot option, which toggles wheather
panic() calls crash_kexec() before panic_notifiers and dump kmsg or after.

The problem is that the commit overlooks panic_on_oops kernel boot option.
If it is enabled, crash_kexec() is called directly without going through
panic() in oops path.

To fix this issue, this patch adds a check to "crash_kexec_post_notifiers"
in the condition of kexec_should_crash().

Also, put a comment in kexec_should_crash() to explain not obvious things
on this patch.

Signed-off-by: HATAYAMA Daisuke <[e-mail]>
Acked-by: Baoquan He <[e-mail]>
Tested-by: Hidehiro Kawai <[e-mail]>
Reviewed-by: Masami Hiramatsu <[e-mail]>
Cc: Vivek Goyal <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Hidehiro Kawai <[e-mail]>
Cc: Baoquan He <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/kernel.h

···
 extern int panic_on_io_nmi;
 extern int panic_on_warn;
 extern int sysctl_panic_on_stackoverflow;
+
+extern bool crash_kexec_post_notifiers;
+
 /*
  * Only to be used by arch init code. If the user over-wrote the default
  * CONFIG_PANIC_TIMEOUT, honor it.

kernel/kexec.c

···
 
 int kexec_should_crash(struct task_struct *p)
 {
+	/*
+	 * If crash_kexec_post_notifiers is enabled, don't run
+	 * crash_kexec() here yet, which must be run after panic
+	 * notifiers in panic().
+	 */
+	if (crash_kexec_post_notifiers)
+		return 0;
+	/*
+	 * There are 4 panic() calls in do_exit() path, each of which
+	 * corresponds to each of these 4 conditions.
+	 */
 	if (in_interrupt() || !p->pid || is_global_init(p) || panic_on_oops)
 		return 1;
 	return 0;

kernel/panic.c

···
 static int pause_on_oops;
 static int pause_on_oops_flag;
 static DEFINE_SPINLOCK(pause_on_oops_lock);
-static bool crash_kexec_post_notifiers;
+bool crash_kexec_post_notifiers;
 int panic_on_warn __read_mostly;
 
 int panic_timeout = CONFIG_PANIC_TIMEOUT;