Selftest failures related to kern_sync_rcu()

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Hi Andrii

I'm getting some selftest failures that all seem to have something to do
with kern_sync_rcu() not being enough to trigger the kernel events that
the selftest expects:

$ ./test_progs | grep FAIL
test_lookup_update:FAIL:map1_leak inner_map1 leaked!
#15/1 lookup_update:FAIL
#15 btf_map_in_map:FAIL
test_exit_creds:FAIL:null_ptr_count unexpected null_ptr_count: actual 0 == expected 0
#123/2 exit_creds:FAIL
#123 task_local_storage:FAIL
test_exit_creds:FAIL:null_ptr_count unexpected null_ptr_count: actual 0 == expected 0
#123/2 exit_creds:FAIL
#123 task_local_storage:FAIL

They are all fixed by adding a sleep(1) after the call(s) to
kern_sync_rcu(), so I'm guessing it's some kind of
timing/synchronisation problem. Is there a particular kernel config
that's needed for the membarrier syscall trick to work? I've tried with
various settings of PREEMPT and that doesn't really seem to make any
difference...

-Toke