perf: Synchronously clean up child events

The orphan cleanup workqueue doesn't always catch orphans, for example,
if they never schedule after they are orphaned. IOW, the event leak is
still very real. It also wouldn't work for kernel counters.

Doing it synchonously is a little hairy due to lock inversion issues,
but is made to work.

Patch based on work by Alexander Shishkin.
Suggested-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
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>
Cc: vince@deater.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/perf_event.h

@@ -634,9 +634,6 @@ struct perf_event_context {
 	int				nr_cgroups;	 /* cgroup evts */
 	void				*task_ctx_data; /* pmu specific data */
 	struct rcu_head			rcu_head;
-
-	struct delayed_work		orphans_remove;
-	bool				orphans_remove_sched;
 };
 
 /*

kernel/events/core.c

@@ -49,8 +49,6 @@
 
 #include <asm/irq_regs.h>
 
-static struct workqueue_struct *perf_wq;
-
 typedef int (*remote_function_f)(void *);
 
 struct remote_function_call {
@@ -1645,45 +1643,11 @@ static void perf_group_detach(struct perf_event *event)
 		perf_event__header_size(tmp);
 }
 
-/*
- * User event without the task.
- */
 static bool is_orphaned_event(struct perf_event *event)
 {
-	return event && event->state == PERF_EVENT_STATE_EXIT;
-}
-
-/*
- * Event has a parent but parent's task finished and it's
- * alive only because of children holding refference.
- */
-static bool is_orphaned_child(struct perf_event *event)
-{
-	return is_orphaned_event(event->parent);
-}
-
-static void orphans_remove_work(struct work_struct *work);
-
-static void schedule_orphans_remove(struct perf_event_context *ctx)
-{
-	if (!ctx->task || ctx->orphans_remove_sched || !perf_wq)
-		return;
-
-	if (queue_delayed_work(perf_wq, &ctx->orphans_remove, 1)) {
-		get_ctx(ctx);
-		ctx->orphans_remove_sched = true;
-	}
-}
-
-static int __init perf_workqueue_init(void)
-{
-	perf_wq = create_singlethread_workqueue("perf");
-	WARN(!perf_wq, "failed to create perf workqueue\n");
-	return perf_wq ? 0 : -1;
+	return event->state == PERF_EVENT_STATE_EXIT;
 }
 
-core_initcall(perf_workqueue_init);
-
 static inline int pmu_filter_match(struct perf_event *event)
 {
 	struct pmu *pmu = event->pmu;
@@ -1744,9 +1708,6 @@ event_sched_out(struct perf_event *event,
 	if (event->attr.exclusive || !cpuctx->active_oncpu)
 		cpuctx->exclusive = 0;
 
-	if (is_orphaned_child(event))
-		schedule_orphans_remove(ctx);
-
 	perf_pmu_enable(event->pmu);
 }
 
@@ -1984,9 +1945,6 @@ event_sched_in(struct perf_event *event,
 	if (event->attr.exclusive)
 		cpuctx->exclusive = 1;
 
-	if (is_orphaned_child(event))
-		schedule_orphans_remove(ctx);
-
 out:
 	perf_pmu_enable(event->pmu);
 
@@ -3369,7 +3327,6 @@ static void __perf_event_init_context(struct perf_event_context *ctx)
 	INIT_LIST_HEAD(&ctx->flexible_groups);
 	INIT_LIST_HEAD(&ctx->event_list);
 	atomic_set(&ctx->refcount, 1);
-	INIT_DELAYED_WORK(&ctx->orphans_remove, orphans_remove_work);
 }
 
 static struct perf_event_context *
@@ -3782,11 +3739,22 @@ static void perf_remove_from_owner(struct perf_event *event)
 
 static void put_event(struct perf_event *event)
 {
-	struct perf_event_context *ctx;
-
 	if (!atomic_long_dec_and_test(&event->refcount))
 		return;
 
+	_free_event(event);
+}
+
+/*
+ * Kill an event dead; while event:refcount will preserve the event
+ * object, it will not preserve its functionality. Once the last 'user'
+ * gives up the object, we'll destroy the thing.
+ */
+int perf_event_release_kernel(struct perf_event *event)
+{
+	struct perf_event_context *ctx;
+	struct perf_event *child, *tmp;
+
 	if (!is_kernel_event(event))
 		perf_remove_from_owner(event);
 
@@ -3811,14 +3779,70 @@ static void put_event(struct perf_event *event)
 	 * At this point we must have event->state == PERF_EVENT_STATE_EXIT,
 	 * either from the above perf_remove_from_context() or through
 	 * perf_event_exit_event().
+	 *
+	 * Therefore, anybody acquiring event->child_mutex after the below
+	 * loop _must_ also see this, most importantly inherit_event() which
+	 * will avoid placing more children on the list.
+	 *
+	 * Thus this guarantees that we will in fact observe and kill _ALL_
+	 * child events.
 	 */
 	WARN_ON_ONCE(event->state != PERF_EVENT_STATE_EXIT);
 
-	_free_event(event);
-}
+again:
+	mutex_lock(&event->child_mutex);
+	list_for_each_entry(child, &event->child_list, child_list) {
 
-int perf_event_release_kernel(struct perf_event *event)
-{
+		/*
+		 * 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 */
 	put_event(event);
 	return 0;
 }
@@ -3829,46 +3853,10 @@ EXPORT_SYMBOL_GPL(perf_event_release_kernel);
  */
 static int perf_release(struct inode *inode, struct file *file)
 {
-	put_event(file->private_data);
+	perf_event_release_kernel(file->private_data);
 	return 0;
 }
 
-/*
- * Remove all orphanes events from the context.
- */
-static void orphans_remove_work(struct work_struct *work)
-{
-	struct perf_event_context *ctx;
-	struct perf_event *event, *tmp;
-
-	ctx = container_of(work, struct perf_event_context,
-			   orphans_remove.work);
-
-	mutex_lock(&ctx->mutex);
-	list_for_each_entry_safe(event, tmp, &ctx->event_list, event_entry) {
-		struct perf_event *parent_event = event->parent;
-
-		if (!is_orphaned_child(event))
-			continue;
-
-		perf_remove_from_context(event, DETACH_GROUP);
-
-		mutex_lock(&parent_event->child_mutex);
-		list_del_init(&event->child_list);
-		mutex_unlock(&parent_event->child_mutex);
-
-		free_event(event);
-		put_event(parent_event);
-	}
-
-	raw_spin_lock_irq(&ctx->lock);
-	ctx->orphans_remove_sched = false;
-	raw_spin_unlock_irq(&ctx->lock);
-	mutex_unlock(&ctx->mutex);
-
-	put_ctx(ctx);
-}
-
 u64 perf_event_read_value(struct perf_event *event, u64 *enabled, u64 *running)
 {
 	struct perf_event *child;
@@ -8719,7 +8707,7 @@ perf_event_exit_event(struct perf_event *child_event,
 	if (parent_event)
 		perf_group_detach(child_event);
 	list_del_event(child_event, child_ctx);
-	child_event->state = PERF_EVENT_STATE_EXIT;
+	child_event->state = PERF_EVENT_STATE_EXIT; /* see perf_event_release_kernel() */
 	raw_spin_unlock_irq(&child_ctx->lock);
 
 	/*
@@ -8977,8 +8965,16 @@ inherit_event(struct perf_event *parent_event,
 	if (IS_ERR(child_event))
 		return child_event;
 
+	/*
+	 * is_orphaned_event() and list_add_tail(&parent_event->child_list)
+	 * must be under the same lock in order to serialize against
+	 * perf_event_release_kernel(), such that either we must observe
+	 * is_orphaned_event() or they will observe us on the child_list.
+	 */
+	mutex_lock(&parent_event->child_mutex);
 	if (is_orphaned_event(parent_event) ||
 	    !atomic_long_inc_not_zero(&parent_event->refcount)) {
+		mutex_unlock(&parent_event->child_mutex);
 		free_event(child_event);
 		return NULL;
 	}
@@ -9026,8 +9022,6 @@ inherit_event(struct perf_event *parent_event,
 	/*
 	 * Link this into the parent event's child list
 	 */
-	WARN_ON_ONCE(parent_event->ctx->parent_ctx);
-	mutex_lock(&parent_event->child_mutex);
 	list_add_tail(&child_event->child_list, &parent_event->child_list);
 	mutex_unlock(&parent_event->child_mutex);