panic: fix the panic_print NMI backtrace setting

Commit 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in
panic_print") introduced a setting for the "panic_print" kernel parameter
to allow users to request a NMI backtrace on panic. Problem is that the
panic_print handling happens after the secondary CPUs are already
disabled, hence this option ended-up being kind of a no-op - kernel skips
the NMI trace in idling CPUs, which is the case of offline CPUs.

Fix it by checking the NMI backtrace bit in the panic_print prior to the
CPU disabling function.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in panic_print")
Signed-off-by: Guilherme G. Piccoli <[e-mail]>
Cc: <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Dave Young <[e-mail]>
Cc: Feng Tang <[e-mail]>
Cc: HATAYAMA Daisuke <[e-mail]>
Cc: Hidehiro Kawai <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Michael Kelley <[e-mail]>
Cc: Petr Mladek <[e-mail]>
Cc: Vivek Goyal <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

kernel/panic.c

···
 		return;
 	}
 
-	if (panic_print & PANIC_PRINT_ALL_CPU_BT)
-		trigger_all_cpu_backtrace();
-
 	if (panic_print & PANIC_PRINT_TASK_INFO)
 		show_state();
 
···
 	if (atomic_inc_return(&warn_count) >= limit && limit)
 		panic("%s: system warned too often (kernel.warn_limit is %d)",
 		      origin, limit);
+}
+
+/*
+ * Helper that triggers the NMI backtrace (if set in panic_print)
+ * and then performs the secondary CPUs shutdown - we cannot have
+ * the NMI backtrace after the CPUs are off!
+ */
+static void panic_other_cpus_shutdown(bool crash_kexec)
+{
+	if (panic_print & PANIC_PRINT_ALL_CPU_BT)
+		trigger_all_cpu_backtrace();
+
+	/*
+	 * Note that smp_send_stop() is the usual SMP shutdown function,
+	 * which unfortunately may not be hardened to work in a panic
+	 * situation. If we want to do crash dump after notifier calls
+	 * and kmsg_dump, we will need architecture dependent extra
+	 * bits in addition to stopping other CPUs, hence we rely on
+	 * crash_smp_send_stop() for that.
+	 */
+	if (!crash_kexec)
+		smp_send_stop();
+	else
+		crash_smp_send_stop();
 }
 
 /**
···
 	 *
 	 * Bypass the panic_cpu check and call __crash_kexec directly.
 	 */
-	if (!_crash_kexec_post_notifiers) {
+	if (!_crash_kexec_post_notifiers)
 		__crash_kexec(NULL);
 
-		/*
-		 * Note smp_send_stop is the usual smp shutdown function, which
-		 * unfortunately means it may not be hardened to work in a
-		 * panic situation.
-		 */
-		smp_send_stop();
-	} else {
-		/*
-		 * If we want to do crash dump after notifier calls and
-		 * kmsg_dump, we will need architecture dependent extra
-		 * works in addition to stopping other CPUs.
-		 */
-		crash_smp_send_stop();
-	}
+	panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
 
 	/*
 	 * Run any panic handlers, including those that might need to