Messages in this thread Patch in this message | | | From | "Eelco Chaudron" <> | Subject | Re: [PATCH 5.9 RT] net: openvswitch: Fix using smp_processor_id() in preemptible code | Date | Mon, 12 Oct 2020 15:36:14 +0200 |
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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
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