Re: [PATCH bpf-next v2] selftests/bpf: fix task_local_storage/exit_creds rcu usage

[sdf@google.com]

On 10/19, Delyan Kratunov 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@fb.com>
> ---
> v1 -> v2:
> Explicit loop counter and MAX_SYNC_RCU_CALLS guard.

>   .../bpf/prog_tests/task_local_storage.c        | 18 +++++++++++++++---
>   .../bpf/progs/task_local_storage_exit_creds.c  |  3 +++
>   2 files changed, 18 insertions(+), 3 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..99a42a2b6e14 100644
> --- a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> +++ b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> @@ -39,7 +39,8 @@ static void test_sys_enter_exit(void)
>   static void test_exit_creds(void)
>   {
>   	struct task_local_storage_exit_creds *skel;
> -	int err;
> +	int err, run_count, sync_rcu_calls = 0;
> +	const int MAX_SYNC_RCU_CALLS = 1000;

>   	skel = task_local_storage_exit_creds__open_and_load();
>   	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
> @@ -53,8 +54,19 @@ 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();
> +		run_count = __atomic_load_n(&skel->bss->run_count, __ATOMIC_SEQ_CST);
> +	} while (run_count == 0 && ++sync_rcu_calls < MAX_SYNC_RCU_CALLS);

Acked-by: Stanislav Fomichev <sdf@google.com>

Might have been easier to do the following instead?

int sync_rcu_calls = 1000;
do {
} while (run_count == 0 && --sync_rcu_calls);


> +
> +	ASSERT_NEQ(sync_rcu_calls, MAX_SYNC_RCU_CALLS,
> +		   "sync_rcu count too high");
> +	ASSERT_NEQ(run_count, 0, "run_count");
>   	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