[PATCH 2/4] samples/cgroup: exit cleanly when the monitored memcg is removed

[Song Hu]

The handler processes IN_MODIFY only.  Since commit eea5d2bb34ba
("kernfs: Send IN_DELETE_SELF and IN_IGNORED") kernfs delivers both
events when the cgroup is removed, but the sample warns "No IN_MODIFY
event, skip it" and returns to poll() forever: after an rmdir the
listener never notices and keeps polling, while a supervisor still
counts it as alive, and the cleanup_memcg_events() call in main() is
unreachable.  The
accompanying selftest, commit 2de27980e1d4 ("selftests: memcg: Add
tests for IN_DELETE_SELF and IN_IGNORED"), exercises exactly this
two-event sequence.

Handle the removal pair by printing a clear message and exiting, and
subscribe IN_DELETE_SELF explicitly.  monitor_events() returns through
the normal path, which also makes the cleanup call reachable again.

Signed-off-by: Song Hu <husong@xxxxxxxxxx>
---
 samples/cgroup/memcg_event_listener.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/samples/cgroup/memcg_event_listener.c b/samples/cgroup/memcg_event_listener.c
index ec7c03fa7887..996bf6fb117c 100644
--- a/samples/cgroup/memcg_event_listener.c
+++ b/samples/cgroup/memcg_event_listener.c
@@ -199,7 +199,7 @@ static int read_memcg_events(struct memcg_events *events, bool show_diff)
 	return ret;
 }
 
-static void process_memcg_events(struct memcg_events *events,
+static bool process_memcg_events(struct memcg_events *events,
 				 struct inotify_event *event)
 {
 	int ret;
@@ -207,19 +207,27 @@ static void process_memcg_events(struct memcg_events *events,
 	if (events->inotify_wd != event->wd) {
 		warnx("Unknown inotify event %d, should be %d", event->wd,
 		      events->inotify_wd);
-		return;
+		return true;
+	}
+
+	if (event->mask & (IN_DELETE_SELF | IN_IGNORED)) {
+		printf("The monitored file %s was removed, exiting\n",
+		       events->path);
+		return false;
 	}
 
 	printf("Received event in %s:\n", events->path);
 
 	if (!(event->mask & IN_MODIFY)) {
 		warnx("No IN_MODIFY event, skip it");
-		return;
+		return true;
 	}
 
 	ret = read_memcg_events(events, /* show_diff = */true);
 	if (ret)
 		warnx("Can't read memcg events");
+
+	return true;
 }
 
 static void monitor_events(struct memcg_events *events)
@@ -247,6 +255,7 @@ static void monitor_events(struct memcg_events *events)
 			struct inotify_event *event;
 			char buffer[INOTIFY_BUFFER_SIZE];
 			ssize_t length;
+			bool keep_going = true;
 
 			length = read(fds[0].fd, buffer, INOTIFY_BUFFER_SIZE);
 			if (length <= 0)
@@ -254,9 +263,15 @@ static void monitor_events(struct memcg_events *events)
 
 			event = (struct inotify_event *)buffer;
 			while (INOTIFY_EVENT_OK(event, length)) {
-				process_memcg_events(events, event);
+				keep_going = process_memcg_events(events,
+								  event);
+				if (!keep_going)
+					break;
 				event = INOTIFY_EVENT_NEXT(event, length);
 			}
+
+			if (!keep_going)
+				return;
 		}
 	}
 }
@@ -291,7 +306,8 @@ static int initialize_memcg_events(struct memcg_events *events,
 	}
 
 	events->inotify_wd = inotify_add_watch(events->inotify_fd,
-					       events->path, IN_MODIFY);
+					       events->path,
+					       IN_MODIFY | IN_DELETE_SELF);
 	if (events->inotify_wd < 0) {
 		warn("Couldn't add monitor in dir %s", events->path);
 		return -EIO;
-- 
2.53.0