notifiers, RCU: Assert that RCU is watching in notify_die()

Low-level arch entries often call notify_die(), and it's easy for
arch code to fail to exit an RCU quiescent state first.  Assert
that we're not quiescent in notify_die().
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1f5fe6c23d5b432a23267102f2d72b787d80fdd8.1435952415.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/notifier.c

@@ -544,6 +544,8 @@ int notrace notify_die(enum die_val val, const char *str,
 		.signr	= sig,
 
 	};
+	rcu_lockdep_assert(rcu_is_watching(),
+			   "notify_die called but RCU thinks we're quiescent");
 	return atomic_notifier_call_chain(&die_chain, val, &args);
 }
 NOKPROBE_SYMBOL(notify_die);