[PATCH 2/5] perf: Destroy event's children on task exit

[Jiri Olsa <jolsa@kernel.org>]

From: Jiri Olsa <jolsa@redhat.com>

When task exits we close:
  1) all events that are installed in task
  2) all events owned by task (via file descriptor)

But we don't close children events of 2) events. Those children
events stay until the child task exits and are useless with the
parent being gone, because we have no way to get to values any
more.

Plus if the event stays installed in task even with the owner task
gone, it runs the perf callback any time the task forks, for no
real reason.

Closing all children events events when the owner task of the
parent event is closed.

Cc: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/events/core.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 71a56ae..37797dd 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7535,6 +7535,32 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
 	put_ctx(child_ctx);
 }
 
+static void perf_event_exit_children(struct perf_event *parent)
+{
+	struct perf_event *child, *tmp;
+
+	mutex_lock(&parent->child_mutex);
+	list_for_each_entry_safe(child, tmp, &parent->child_list,
+				 child_list) {
+		struct perf_event_context *child_ctx = child->ctx;
+
+		/*
+		 * Child events got removed from child_list under
+		 * child_mutex and then freed. So it's safe to access
+		 * childs context in here, because the child holds
+		 * context ref.
+		 */
+		mutex_lock(&child_ctx->mutex);
+		perf_remove_from_context(child, true);
+		mutex_unlock(&child_ctx->mutex);
+
+		list_del_init(&child->child_list);
+		put_event(parent);
+		free_event(child);
+	}
+	mutex_unlock(&parent->child_mutex);
+}
+
 /*
  * When a child task exits, feed back event values to parent events.
  */
@@ -7555,6 +7581,7 @@ void perf_event_exit_task(struct task_struct *child)
 		 */
 		smp_wmb();
 		event->owner = NULL;
+		perf_event_exit_children(event);
 	}
 	mutex_unlock(&child->perf_event_mutex);
 
-- 
1.8.3.1