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

[Eelco Chaudron]

On 12 Oct 2020, at 10:21, Sebastian Andrzej Siewior wrote:

> On 2020-10-12 10:14:42 [+0200], Eelco Chaudron wrote:
> > 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.
>
> _no_ As explained above, this is not RT specific.
> What guaranties that you don't have two flow_lookup() invocations on 
> the
> same CPU? You are using u64_stats_update_begin() which must not be
> preempted. This means even if preemption is disabled you must not have
> another invocation in BH context. This is due to the
> write_seqcount_begin() in u64_stats_update_begin().
> If preemption / CPU migration is not a problem in the above part, you
> can use annotation to disable the warning that led to the warning. But
> the u64_stats invocation looks still problematic.
>
> > So maybe this needs a get_cpu() instead of the light variant in the 
> > RT case?

Hi Sebastian,

I was not reading the splat correctly and thought it was from the NAPI 
path but it looks like it's from the netlink part. I think this could be 
fixed with the following patch, so both paths, NAPI, and netlink become 
non-preemptive:

--- a/net/openvswitch/flow_table.c
+++ b/net/openvswitch/flow_table.c
@@ -850,9 +850,14 @@ struct sw_flow *ovs_flow_tbl_lookup(struct 
flow_table *tbl,
        struct mask_array *ma = rcu_dereference_ovsl(tbl->mask_array);
        u32 __always_unused n_mask_hit;
        u32 __always_unused n_cache_hit;
+       struct sw_flow *flow;
        u32 index = 0;

-       return flow_lookup(tbl, ti, ma, key, &n_mask_hit, &n_cache_hit, 
&index);
+       preempt_disable();
+       flow = flow_lookup(tbl, ti, ma, key, &n_mask_hit, &n_cache_hit, 
&index);
+       preempt_enable();
+
+       return flow;
 }

 struct sw_flow *ovs_flow_tbl_lookup_exact(struct flow_table *tbl,

Note that choosing to do this in ovs_flow_tbl_lookup() and not 
flow_lookup() allows the fast-path, through NAPI, not having to call 
preempt_disable().

Any comments? If not I can sent a proper patch trough netdev.

//Eelco