Re: [PATCH bpf-next] selftests/bpf: fix task_local_storage/exit_creds rcu usage

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On Tue, Oct 18, 2022 at 4:25 PM Delyan Kratunov <delyank@xxxxxxxx> wrote:
>
> BPF CI has revealed flakiness in the task_local_storage/exit_creds test.
> The failure point in CI [1] is that null_ptr_count is equal to 0,
> which indicates that the program hasn't run yet. This points to the
> kern_sync_rcu (sys_membarrier -> synchronize_rcu underneath) not
> waiting sufficiently.
>
> Indeed, synchronize_rcu only waits for read-side sections that started
> before the call. If the program execution starts *during* the
> synchronize_rcu invocation (due to, say, preemption), the test won't
> wait long enough.
>
> As a speculative fix, make the synchornize_rcu calls in a loop until
> an explicit run counter has gone up.
>
>   [1]: https://github.com/kernel-patches/bpf/actions/runs/3268263235/jobs/5374940791
>
> Signed-off-by: Delyan Kratunov <delyank@xxxxxx>
> ---
>  .../selftests/bpf/prog_tests/task_local_storage.c     | 11 +++++++++--
>  .../bpf/progs/task_local_storage_exit_creds.c         |  3 +++
>  2 files changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> index 035c263aab1b..4e2e3293c914 100644
> --- a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> +++ b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> @@ -53,8 +53,15 @@ static void test_exit_creds(void)
>         if (CHECK_FAIL(system("ls > /dev/null")))
>                 goto out;
>
> -       /* sync rcu to make sure exit_creds() is called for "ls" */
> -       kern_sync_rcu();
> +       /* kern_sync_rcu is not enough on its own as the read section we want
> +        * to wait for may start after we enter synchronize_rcu, so our call
> +        * won't wait for the section to finish. Loop on the run counter
> +        * as well to ensure the program has run.
> +        */
> +       do {
> +               kern_sync_rcu();
> +       } while (__atomic_load_n(&skel->bss->run_count, __ATOMIC_SEQ_CST) == 0);

let's also add some big enough max counter here to avoid being stuck
if something about test breaks and we actually never trigger the
program?

> +
>         ASSERT_EQ(skel->bss->valid_ptr_count, 0, "valid_ptr_count");
>         ASSERT_NEQ(skel->bss->null_ptr_count, 0, "null_ptr_count");
>  out:
> diff --git a/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c b/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c
> index 81758c0aef99..41d88ed222ff 100644
> --- a/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c
> +++ b/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c
> @@ -14,6 +14,7 @@ struct {
>         __type(value, __u64);
>  } task_storage SEC(".maps");
>
> +int run_count = 0;
>  int valid_ptr_count = 0;
>  int null_ptr_count = 0;
>
> @@ -28,5 +29,7 @@ int BPF_PROG(trace_exit_creds, struct task_struct *task)
>                 __sync_fetch_and_add(&valid_ptr_count, 1);
>         else
>                 __sync_fetch_and_add(&null_ptr_count, 1);
> +
> +       __sync_fetch_and_add(&run_count, 1);
>         return 0;
>  }
> --
> 2.37.3



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