[PATCH 1/7] perf: Ensure we do not free event->parent before event

[Oleg Nesterov]

__perf_event_exit_task() does:

	sync_child_event(event)
	free_event(event)

sync_child_event() does put_event(event->parent) which can actually
free ->parent. This means that event->destroy(event) is called with
->parent pointing to nowhere.

perf_free_event() does put_parent(parent) before free_event(child)
too.

Afaics, currently this is fine. But the tracing "subclasses" (like
trace_uprobe) may want to track the events and their parents, and even
the fact that parent->destroy() is called before child->destroy() can
complicate this.

Move this put_event() from sync_child_event() to its single caller,
__perf_event_exit_task(). Change perf_free_event() the same way.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/events/core.c |   24 ++++++++++++------------
 1 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 301079d..1b2e516 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6787,12 +6787,6 @@ static void sync_child_event(struct perf_event *child_event,
 	mutex_lock(&parent_event->child_mutex);
 	list_del_init(&child_event->child_list);
 	mutex_unlock(&parent_event->child_mutex);
-
-	/*
-	 * Release the parent event, if this was the last
-	 * reference to it.
-	 */
-	put_event(parent_event);
 }
 
 static void
@@ -6800,7 +6794,9 @@ __perf_event_exit_task(struct perf_event *child_event,
 			 struct perf_event_context *child_ctx,
 			 struct task_struct *child)
 {
-	if (child_event->parent) {
+	struct perf_event *parent_event = child_event->parent;
+
+	if (parent_event) {
 		raw_spin_lock_irq(&child_ctx->lock);
 		perf_group_detach(child_event);
 		raw_spin_unlock_irq(&child_ctx->lock);
@@ -6813,9 +6809,14 @@ __perf_event_exit_task(struct perf_event *child_event,
 	 * that are still around due to the child reference. These
 	 * events need to be zapped.
 	 */
-	if (child_event->parent) {
+	if (parent_event) {
 		sync_child_event(child_event, child);
 		free_event(child_event);
+		/*
+		 * Release the parent event, if this was the last
+		 * reference to it.
+		 */
+		put_event(parent_event);
 	}
 }
 
@@ -6867,8 +6868,7 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
 	 * We can recurse on the same lock type through:
 	 *
 	 *   __perf_event_exit_task()
-	 *     sync_child_event()
-	 *       put_event()
+	 *       put_event(parent_event)
 	 *         mutex_lock(&ctx->mutex)
 	 *
 	 * But since its the parent context it won't be the same instance.
@@ -6937,11 +6937,11 @@ static void perf_free_event(struct perf_event *event,
 	list_del_init(&event->child_list);
 	mutex_unlock(&parent->child_mutex);
 
-	put_event(parent);
-
 	perf_group_detach(event);
 	list_del_event(event, ctx);
 	free_event(event);
+
+	put_event(parent);
 }
 
 /*
-- 
1.5.5.1

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