selftests/bpf: fix task_local_storage/exit_creds rcu usage
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@meta.com>
Link: https://lore.kernel.org/r/156d4ef82275a074e8da8f4cffbd01b0c1466493.camel@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>