Merge tag 'perf-urgent-2024-06-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull perf event fix from Ingo Molnar:
 "Fix race between perf_event_free_task() and perf_event_release_kernel()
  that can result in missed wakeups and hung tasks"

* tag 'perf-urgent-2024-06-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  perf/core: Fix missing wakeup when waiting for context reference

Author: torvalds

kernel/events/core.c

@@ -5384,6 +5384,7 @@ int perf_event_release_kernel(struct perf_event *event)
 again:
 	mutex_lock(&event->child_mutex);
 	list_for_each_entry(child, &event->child_list, child_list) {
+		void *var = NULL;
 
 		/*
 		 * Cannot change, child events are not migrated, see the
@@ -5424,11 +5425,23 @@ int perf_event_release_kernel(struct perf_event *event)
 			 * this can't be the last reference.
 			 */
 			put_event(event);
+		} else {
+			var = &ctx->refcount;
 		}
 
 		mutex_unlock(&event->child_mutex);
 		mutex_unlock(&ctx->mutex);
 		put_ctx(ctx);
+
+		if (var) {
+			/*
+			 * If perf_event_free_task() has deleted all events from the
+			 * ctx while the child_mutex got released above, make sure to
+			 * notify about the preceding put_ctx().
+			 */
+			smp_mb(); /* pairs with wait_var_event() */
+			wake_up_var(var);
+		}
 		goto again;
 	}
 	mutex_unlock(&event->child_mutex);