[LTP] [PATCH v2] fanotify22.c: handle multiple asynchronous error events

[Wei Gao]

Since the introduction of the asynchronous fserror reporting framework
(kernel commit 81d2e13a57c9), fanotify22 has encountered sporadic failures
due to the non-deterministic nature of event delivery and merging:

1) tcase3 failure: A race condition occurs when the test reads the
   notification fd between two events. Adding a short delay
   (usleep) ensures all events are dispatched and ready before the
   read() call.

2) tcase4 failure: The kernel may deliver errors as independent events
   instead of a single merged event, since different worker kthread can
   end up generating each event so they won't be merged. As suggested by
   Jan Kara, this patch introduces a consolidate_events() helper. It iterates
   through the event buffer, accumulates the error_count from all independent
   events, and updates the first event's count in-place.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202602042124.87bd00e3-lkp@intel.com
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wei Gao <wegao@suse.com>
---
 .../kernel/syscalls/fanotify/fanotify22.c     | 37 ++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/testcases/kernel/syscalls/fanotify/fanotify22.c b/testcases/kernel/syscalls/fanotify/fanotify22.c
index 6578474a7..4c0fd31b1 100644
--- a/testcases/kernel/syscalls/fanotify/fanotify22.c
+++ b/testcases/kernel/syscalls/fanotify/fanotify22.c
@@ -88,7 +88,6 @@ static void trigger_bad_link_lookup(void)
 			ret, BAD_LINK, errno, EUCLEAN);
 }
 
-
 static void tcase3_trigger(void)
 {
 	trigger_bad_link_lookup();
@@ -138,6 +137,38 @@ static struct test_case {
 	}
 };
 
+static size_t consolidate_events(char *buf, size_t len)
+{
+	struct fanotify_event_metadata *metadata, *first = NULL;
+	struct fanotify_event_info_error *first_info = NULL;
+	unsigned int total_count = 0;
+	int event_num = 0;
+
+	for (metadata = (struct fanotify_event_metadata *)buf;
+			FAN_EVENT_OK(metadata, len);
+			metadata = FAN_EVENT_NEXT(metadata, len)) {
+
+		event_num++;
+		struct fanotify_event_info_error *info = get_event_info_error(metadata);
+
+		if (info) {
+			if (!first) {
+				first = metadata;
+				first_info = info;
+			}
+			total_count += info->error_count;
+
+			tst_res(TINFO, "Event [%d]: errno=%d, error_count=%d",
+					event_num, info->error, info->error_count);
+		}
+	}
+
+	if (first_info)
+		first_info->error_count = total_count;
+
+	return (first) ? first->event_len : 0;
+}
+
 static int check_error_event_info_fid(struct fanotify_event_info_fid *fid,
 				 const struct test_case *ex)
 {
@@ -255,7 +286,11 @@ static void do_test(unsigned int i)
 
 	tcase->trigger_error();
 
+	/* Wait for asynchronous kworker threads to dispatch events */
+	usleep(100000);
+
 	read_len = SAFE_READ(0, fd_notify, event_buf, BUF_SIZE);
+	read_len = consolidate_events(event_buf, read_len);
 
 	SAFE_FANOTIFY_MARK(fd_notify, FAN_MARK_REMOVE|FAN_MARK_FILESYSTEM,
 			   FAN_FS_ERROR, AT_FDCWD, MOUNT_PATH);
-- 
2.52.0