+ /*
+ * Cannot change, child events are not migrated, see the
+ * comment with perf_event_ctx_lock_nested().
+ */
+ ctx = lockless_dereference(child->ctx);
+ /*
+ * Since child_mutex nests inside ctx::mutex, we must jump
+ * through hoops. We start by grabbing a reference on the ctx.
+ *
+ * Since the event cannot get freed while we hold the
+ * child_mutex, the context must also exist and have a !0
+ * reference count.
+ */
+ get_ctx(ctx);
+
+ /*
+ * Now that we have a ctx ref, we can drop child_mutex, and
+ * acquire ctx::mutex without fear of it going away. Then we
+ * can re-acquire child_mutex.
+ */
+ mutex_unlock(&event->child_mutex);
+ mutex_lock(&ctx->mutex);
+ mutex_lock(&event->child_mutex);
+
+ /*
+ * Now that we hold ctx::mutex and child_mutex, revalidate our
+ * state, if child is still the first entry, it didn't get freed
+ * and we can continue doing so.
+ */
+ tmp = list_first_entry_or_null(&event->child_list,
+ struct perf_event, child_list);
+ if (tmp == child) {
+ perf_remove_from_context(child, DETACH_GROUP);
+ list_del(&child->child_list);
+ free_event(child);
+ /*
+ * This matches the refcount bump in inherit_event();
+ * this can't be the last reference.
+ */
+ put_event(event);
+ }
+
+ mutex_unlock(&event->child_mutex);
+ mutex_unlock(&ctx->mutex);
+ put_ctx(ctx);
+ goto again;
+ }
+ mutex_unlock(&event->child_mutex);
+
+ /* Must be the last reference */