Messages in this thread | | | From | "Eelco Chaudron" <> | 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 |
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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
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