[PATCH v3] selftests/user_events: wait for deferred event teardown after unregister

[Michael Bommarito <michael.bommarito@gmail.com>]

Unregistering a user event now defers the drop of the enabler's event
reference (and the freeing of the enabler) past an RCU grace period. As a
result DIAG_IOCSDEL can transiently fail with -EBUSY while that last
reference is still being dropped, where it previously succeeded
immediately.

Two tests assumed the delete takes effect the instant the unregister
returns:

  - abi_test "flags" deletes the event right after disabling it.
  - perf_test's fixture teardown clear() deletes __test_event before the
    next test registers the same name; a stale event makes the following
    registration fail with -EADDRINUSE.

Retry the delete until it succeeds (or the event is already gone) with a
bounded wait, matching the existing wait_for_delete() idiom in the same
suite, so the tests are robust to the deferred teardown.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
This resends only the selftest patch; the tracing/user_events fix that
was patch 1/2 of the v2 series is unchanged and being applied separately.

v2 -> v3:
  - abi_test wait_for_event_delete(): only retry the delete on -EBUSY,
    treat an already-deleted event (-ENOENT) as success, and return any
    other error immediately instead of spinning for the full 10s timeout.
    perf_test's clear() already discriminated on errno this way; the two
    now match. Reported by Sashiko automated review.

 .../testing/selftests/user_events/abi_test.c  | 29 ++++++++++++++++++-
 .../testing/selftests/user_events/perf_test.c | 26 +++++++++++++++--
 2 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/user_events/abi_test.c b/tools/testing/selftests/user_events/abi_test.c
index 85892b3b719cc..b71813eaf5c04 100644
--- a/tools/testing/selftests/user_events/abi_test.c
+++ b/tools/testing/selftests/user_events/abi_test.c
@@ -132,6 +132,33 @@ static int event_delete(void)
 	return ret;
 }
 
+/*
+ * Deleting an event drops its last reference, but an unregister may defer
+ * that put (and the freeing of the associated enabler) past an RCU grace
+ * period. The delete can therefore transiently fail with -EBUSY while the
+ * previous reference is still being dropped. Retry only on that transient
+ * failure; treat an already-deleted event (-ENOENT) as success and return
+ * any other error immediately rather than spinning for the full timeout.
+ */
+static int wait_for_event_delete(void)
+{
+	int i, ret;
+
+	for (i = 0; i < 10000; ++i) {
+		ret = event_delete();
+
+		if (ret == 0 || errno == ENOENT)
+			return 0;
+
+		if (errno != EBUSY)
+			return ret;
+
+		usleep(1000);
+	}
+
+	return ret;
+}
+
 static int reg_enable_multi(void *enable, int size, int bit, int flags,
 			    char *args)
 {
@@ -262,7 +289,7 @@ TEST_F(user, flags) {
 	ASSERT_TRUE(event_exists());
 
 	/* Ensure we can delete it */
-	ASSERT_EQ(0, event_delete());
+	ASSERT_EQ(0, wait_for_event_delete());
 
 	/* USER_EVENT_REG_MAX or above is not allowed */
 	ASSERT_EQ(-1, reg_enable_flags(&self->check, sizeof(int), 0,
diff --git a/tools/testing/selftests/user_events/perf_test.c b/tools/testing/selftests/user_events/perf_test.c
index cafec0e52eb31..5727cb5b914cf 100644
--- a/tools/testing/selftests/user_events/perf_test.c
+++ b/tools/testing/selftests/user_events/perf_test.c
@@ -85,6 +85,7 @@ static int get_offset(void)
 static int clear(int *check)
 {
 	struct user_unreg unreg = {0};
+	int i, ret;
 
 	unreg.size = sizeof(unreg);
 	unreg.disable_bit = 31;
@@ -99,13 +100,32 @@ static int clear(int *check)
 		if (errno != ENOENT)
 			return -1;
 
-	if (ioctl(fd, DIAG_IOCSDEL, "__test_event") == -1)
-		if (errno != ENOENT)
+	/*
+	 * Deleting the event drops its last reference, but the unregister
+	 * above defers that put (and the freeing of the enabler) past an RCU
+	 * grace period. The delete can therefore transiently fail with -EBUSY
+	 * until that reference is dropped. Retry for up to ~10 seconds so the
+	 * event is actually gone before the next test registers the same name.
+	 */
+	for (i = 0; i < 10000; ++i) {
+		ret = ioctl(fd, DIAG_IOCSDEL, "__test_event");
+
+		if (ret == 0 || errno == ENOENT) {
+			ret = 0;
+			break;
+		}
+
+		if (errno != EBUSY) {
+			close(fd);
 			return -1;
+		}
+
+		usleep(1000);
+	}
 
 	close(fd);
 
-	return 0;
+	return ret;
 }
 
 FIXTURE(user) {
-- 
2.53.0