linux-kernel - Re: [PATCH 5.9 RT] net: openvswitch: Fix using smp_processor

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Date:   Mon, 12 Oct 2020 10:21:29 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Eelco Chaudron <echaudro@...hat.com>
Cc:     Juri Lelli <juri.lelli@...hat.com>, tglx@...utronix.de,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        bristot@...hat.com, williams@...hat.com, atheurer@...hat.com
Subject: Re: [PATCH 5.9 RT] net: openvswitch: Fix using smp_processor_id() in
 preemptible code

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?

> //Eelco

Sebastian