[PATCH 3/4] samples/cgroup: arm the inotify watch before reading the baseline

[Song Hu]

The listener reads the initial counter state first and arms the
inotify watch afterwards, so any event firing in between is lost:
it is already included in the baseline (no diff will ever be reported)
and no watch existed to queue a notification.

Arm the watch first, then read the baseline.

Signed-off-by: Song Hu <husong@xxxxxxxxxx>
---
 samples/cgroup/memcg_event_listener.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/samples/cgroup/memcg_event_listener.c b/samples/cgroup/memcg_event_listener.c
index 996bf6fb117c..c23af43bf6cc 100644
--- a/samples/cgroup/memcg_event_listener.c
+++ b/samples/cgroup/memcg_event_listener.c
@@ -293,12 +293,6 @@ static int initialize_memcg_events(struct memcg_events *events,
 		return ret;
 	}
 
-	ret = read_memcg_events(events, /* show_diff = */false);
-	if (ret) {
-		warnx("Failed to read initial memcg events state (%d)", ret);
-		return ret;
-	}
-
 	events->inotify_fd = inotify_init();
 	if (events->inotify_fd < 0) {
 		warn("Failed to setup new inotify device");
@@ -313,6 +307,17 @@ static int initialize_memcg_events(struct memcg_events *events,
 		return -EIO;
 	}
 
+	/*
+	 * Read the initial state only after the watch is armed, so no
+	 * event can slip through the window between the baseline read and
+	 * the watch creation.
+	 */
+	ret = read_memcg_events(events, /* show_diff = */false);
+	if (ret) {
+		warnx("Failed to read initial memcg events state (%d)", ret);
+		return ret;
+	}
+
 	printf("Initialized MEMCG events with counters:\n");
 	print_memcg_counters(&events->counters);
 
-- 
2.53.0