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 [thread overview]
Message-ID: <65BBD0B4-2A74-421A-BF81-357CD5F84747@redhat.com> (raw)
In-Reply-To: <20201009154116.a4fcrrm7flxonidd@linutronix.de>
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
next prev parent reply other threads:[~2020-10-12 8:14 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-09 12:47 [PATCH 5.9 RT] net: openvswitch: Fix using smp_processor_id() in preemptible code Juri Lelli
2020-10-09 15:01 ` Juri Lelli
2020-10-09 15:41 ` Sebastian Andrzej Siewior
2020-10-12 8:14 ` Eelco Chaudron [this message]
2020-10-12 8:21 ` Sebastian Andrzej Siewior
2020-10-12 13:36 ` Eelco Chaudron
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