On 2020-10-09 14:47:59 [+0200], Juri Lelli wrote:

This happens because openvswitch/flow_table::flow_lookup() accesses
per-cpu data while being preemptible (and migratable).

Fix it by adding get/put_cpu_light(), so that, even if preempted, the
task executing this code is not migrated (operation is also guarded by
ovs_mutex mutex).

This warning is not limited to PREEMPT_RT it also present upstream since
commit
   eac87c413bf97 ("net: openvswitch: reorder masks array based on usage")

You should be able to reproduce it there, too.
The path ovs_flow_tbl_lookup() -> flow_lookup() is guarded by ovs_lock()
I can't say that this true for
   ovs_vport_receive() -> ovs_dp_process_packet() ->
   ovs_flow_tbl_lookup_stats() -> flow_lookup()

(means I don't know but it looks like coming from NAPI).

Which means u64_stats_update_begin() could have two writers. This must
not happen.
There are two reader which do u64_stats_fetch_begin_irq(). Disabling
interrupts makes no sense since they perform cross-CPU access.

-> You need to ensure that there is only one writer at a time.

If mask_array gains a spinlock_t for writer protection then you can
acquire the lock prio grabbing ->masks_usage_cntr. But as of now there
is one `ma->syncp'.

Sebastian