[PATCH bpf-next] selftests/bpf: Fix map_kptr test failure

[Yonghong Song <yonghong.song@linux.dev>]

On my arm64 machine, I get the following failure:
  ...
  tester_init:PASS:tester_log_buf 0 nsec
  process_subtest:PASS:obj_open_mem 0 nsec
  process_subtest:PASS:specs_alloc 0 nsec
  serial_test_map_kptr:PASS:rcu_tasks_trace_gp__open_and_load 0 nsec
  ...
  test_map_kptr_success:PASS:map_kptr__open_and_load 0 nsec
  test_map_kptr_success:PASS:test_map_kptr_ref1 refcount 0 nsec
  test_map_kptr_success:FAIL:test_map_kptr_ref1 retval unexpected error: 2 (errno 2)
  test_map_kptr_success:PASS:test_map_kptr_ref2 refcount 0 nsec
  test_map_kptr_success:FAIL:test_map_kptr_ref2 retval unexpected error: 1 (errno 2)
  ...
  #201/21  map_kptr/success-map:FAIL

In serial_test_map_kptr(), before test_map_kptr_success(), one
kern_sync_rcu() is used to have some delay for freeing the map.
But in my environment, one kern_sync_rcu() seems not enough and
caused the test failure.

In bpf_map_free_in_work() in syscall.c, the queue time for
  queue_work(system_dfl_wq, &map->work)
may be longer than expected. This may cause the test failure
since test_map_kptr_success() expects all previous maps having been freed.

In stead of one kern_sync_rcu() before test_map_kptr_success(),
I added two more kern_sync_rcu() to have a longer delay and
the test succeeded.

Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 tools/testing/selftests/bpf/prog_tests/map_kptr.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/map_kptr.c b/tools/testing/selftests/bpf/prog_tests/map_kptr.c
index 8743df599567..f9cfc4d3153c 100644
--- a/tools/testing/selftests/bpf/prog_tests/map_kptr.c
+++ b/tools/testing/selftests/bpf/prog_tests/map_kptr.c
@@ -148,11 +148,15 @@ void serial_test_map_kptr(void)
 
 		ASSERT_OK(kern_sync_rcu_tasks_trace(skel), "sync rcu_tasks_trace");
 		ASSERT_OK(kern_sync_rcu(), "sync rcu");
+		ASSERT_OK(kern_sync_rcu(), "sync rcu");
+		ASSERT_OK(kern_sync_rcu(), "sync rcu");
 		/* Observe refcount dropping to 1 on bpf_map_free_deferred */
 		test_map_kptr_success(false);
 
 		ASSERT_OK(kern_sync_rcu_tasks_trace(skel), "sync rcu_tasks_trace");
 		ASSERT_OK(kern_sync_rcu(), "sync rcu");
+		ASSERT_OK(kern_sync_rcu(), "sync rcu");
+		ASSERT_OK(kern_sync_rcu(), "sync rcu");
 		/* Observe refcount dropping to 1 on synchronous delete elem */
 		test_map_kptr_success(true);
 	}
-- 
2.47.3