perf: Ensure we call add_event_to_ctx() with the right locks held

Even though we call it from the inherit path, where the child is
not yet accessible, we need to hold ctx->lock, add_event_to_ctx()
assumes IRQs are disabled.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -5913,6 +5913,7 @@ inherit_event(struct perf_event *parent_event,
 	      struct perf_event_context *child_ctx)
 {
 	struct perf_event *child_event;
+	unsigned long flags;
 
 	/*
 	 * Instead of creating recursive hierarchies of events,
@@ -5957,7 +5958,9 @@ inherit_event(struct perf_event *parent_event,
 	/*
 	 * Link it up in the child's context:
 	 */
+	raw_spin_lock_irqsave(&child_ctx->lock, flags);
 	add_event_to_ctx(child_event, child_ctx);
+	raw_spin_unlock_irqrestore(&child_ctx->lock, flags);
 
 	/*
 	 * Get a reference to the parent filp - we will fput it