+/*
+ * Because of perf_event::ctx migration in sys_perf_event_open::move_group we
+ * need some magic.
+ *
+ * Those places that change perf_event::ctx will hold both
+ * perf_event_ctx::mutex of the 'old' and 'new' ctx value.
+ *
+ * Lock ordering is by mutex address. There is one other site where
+ * perf_event_context::mutex nests and that is put_event(). But remember that
+ * that is a parent<->child context relation, and migration does not affect
+ * children, therefore these two orderings should not interact.
+ *
+ * The change in perf_event::ctx does not affect children (as claimed above)
+ * because the sys_perf_event_open() case will install a new event and break
+ * the ctx parent<->child relation.
+ *
+ * The places that change perf_event::ctx will issue:
+ *
+ * perf_remove_from_context();
+ * synchronize_rcu();
+ * perf_install_in_context();
+ *
+ * to affect the change. The remove_from_context() + synchronize_rcu() should
+ * quiesce the event, after which we can install it in the new location. This
+ * means that only external vectors (perf_fops, prctl) can perturb the event
+ * while in transit. Therefore all such accessors should also acquire
+ * perf_event_context::mutex to serialize against this.
+ *
+ * However; because event->ctx can change while we're waiting to acquire
+ * ctx->mutex we must be careful and use the below perf_event_ctx_lock()
+ * function.
+ *
+ * Lock order:
+ * task_struct::perf_event_mutex
+ * perf_event_context::mutex
+ * perf_event_context::lock
+ * perf_event::child_mutex;
+ * perf_event::mmap_mutex
+ * mmap_sem
+ */
+static struct perf_event_context *perf_event_ctx_lock(struct perf_event *event)
+{
+ struct perf_event_context *ctx;
+
+again:
+ rcu_read_lock();
+ ctx = ACCESS_ONCE(event->ctx);
+ if (!atomic_inc_not_zero(&ctx->refcount)) {
+ rcu_read_unlock();
+ goto again;
+ }
+ rcu_read_unlock();
+
+ mutex_lock(&ctx->mutex);
+ if (event->ctx != ctx) {
+ mutex_unlock(&ctx->mutex);
+ put_ctx(ctx);
+ goto again;
+ }
+
+ return ctx;
+}
+
+static void perf_event_ctx_unlock(struct perf_event *event,
+ struct perf_event_context *ctx)
+{
+ mutex_unlock(&ctx->mutex);
+ put_ctx(ctx);
+}
+