perf: Add more assertions

Try to trigger warnings before races do damage.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -263,6 +263,15 @@ static void event_function_call(struct perf_event *event, event_f func, void *da
 		.data = data,
 	};
 
+	if (!event->parent) {
+		/*
+		 * If this is a !child event, we must hold ctx::mutex to
+		 * stabilize the the event->ctx relation. See
+		 * perf_event_ctx_lock().
+		 */
+		lockdep_assert_held(&ctx->mutex);
+	}
+
 	if (!task) {
 		cpu_function_call(event->cpu, event_function, &efs);
 		return;