Re: Selftest failures related to kern_sync_rcu()

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Apr 8, 2021 at 12:34 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> 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...
>

If you check kern_sync_rcu(), it relies on membarrier() syscall
(passing cmd = MEMBARRIER_CMD_SHARED == MEMBARRIER_CMD_GLOBAL).
Now, looking at kernel sources:
  - CONFIG_MEMBARRIER should be enabled for that syscall;
  - it has some extra conditions:

           case MEMBARRIER_CMD_GLOBAL:
                /* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */
                if (tick_nohz_full_enabled())
                        return -EINVAL;
                if (num_online_cpus() > 1)
                        synchronize_rcu();
                return 0;

Could it be that one of those conditions is not satisfied?


> -Toke
>