Hi,
On 12/5/2023 2:04 PM, Yonghong Song wrote:
> With previous patch, one of subtests in test_btf_id becomes
> flaky and may fail. The following is a failing example:
>
> Error: #26 btf
> Error: #26/174 btf/BTF ID
> Error: #26/174 btf/BTF ID
> btf_raw_create:PASS:check 0 nsec
> btf_raw_create:PASS:check 0 nsec
> test_btf_id:PASS:check 0 nsec
> ...
> test_btf_id:PASS:check 0 nsec
> test_btf_id:FAIL:check BTF lingersdo_test_get_info:FAIL:check failed: -1
>
> The test tries to prove a btf_id not available after the map is closed.
> But btf_id is freed only after workqueue and a rcu grace period, compared
> to previous case just after a rcu grade period.
It is not accurate. Before applying the patch, the btf_id will be
released in btf_put() and there is no RCU grace period involved. After
applying the patch, the btf_id will be released after the running of
bpf_map_free_deferred kworker.
>
> To fix the flaky test, I added a kern_sync_rcu() after closing map and
> before querying btf id availability, essentially ensuring a rcu grace
> period in the kernel, which seems making the test happy.
kern_sync_rcu() doesn't guarantee the bpf_map_free_deferred kworker will
complete, so why not remove the test case instead ?
>
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
> tools/testing/selftests/bpf/prog_tests/btf.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/btf.c b/tools/testing/selftests/bpf/prog_tests/btf.c
> index 8fb4a04fbbc0..7feb4223bbac 100644
> --- a/tools/testing/selftests/bpf/prog_tests/btf.c
> +++ b/tools/testing/selftests/bpf/prog_tests/btf.c
> @@ -4629,6 +4629,7 @@ static int test_btf_id(unsigned int test_num)
>
> /* The map holds the last ref to BTF and its btf_id */
> close(map_fd);
> + kern_sync_rcu();
> map_fd = -1;
> btf_fd[0] = bpf_btf_get_fd_by_id(map_info.btf_id);
> if (CHECK(btf_fd[0] >= 0, "BTF lingers")) {
