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From:   "Eelco Chaudron" <echaudro@redhat.com>
To:     "Sebastian Andrzej Siewior" <bigeasy@linutronix.de>
Cc:     "Juri Lelli" <juri.lelli@redhat.com>, tglx@linutronix.de,
        linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        bristot@redhat.com, williams@redhat.com, atheurer@redhat.com
Subject: Re: [PATCH 5.9 RT] net: openvswitch: Fix using smp_processor_id() in
 preemptible code
Date:   Mon, 12 Oct 2020 10:14:42 +0200
Message-ID: <65BBD0B4-2A74-421A-BF81-357CD5F84747@redhat.com>
In-Reply-To: <20201009154116.a4fcrrm7flxonidd@linutronix.de>
References: <20201009124759.592550-1-juri.lelli@redhat.com>
 <20201009154116.a4fcrrm7flxonidd@linutronix.de>
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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