Re: [PATCH 5.9 RT] net: openvswitch: Fix using smp_processor_id() in preemptible code

[Eelco Chaudron]

On 9 Oct 2020, at 17:41, Sebastian Andrzej Siewior wrote:

> 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'.

I’m not too familiar with the RT kernel, but in the none RT kernel, 
this function is called in run to completion parts only, hence does not 
need a lock. Actually, this was designed in such a way that it does not 
need a lock at all.

So maybe this needs a get_cpu() instead of the light variant in the RT 
case?

//Eelco