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Date:   Thu, 15 Oct 2020 13:22:29 +0200
From:   Ilya Maximets <i.maximets@....org>
To:     Ilya Maximets <i.maximets@....org>,
        Eelco Chaudron <echaudro@...hat.com>
Cc:     netdev@...r.kernel.org, dev@...nvswitch.org, bigeasy@...utronix.de,
        jlelli@...hat.com, kuba@...nel.org, pabeni@...hat.com,
        davem@...emloft.net
Subject: Re: [ovs-dev] [PATCH net v2] net: openvswitch: fix to make sure
 flow_lookup() is not preempted

On 10/15/20 1:04 PM, Ilya Maximets wrote:
> On 10/15/20 12:54 PM, Eelco Chaudron wrote:
>>
>>
>> On 15 Oct 2020, at 12:27, Ilya Maximets wrote:
>>
>>> On 10/15/20 11:46 AM, Eelco Chaudron wrote:
>>>> The flow_lookup() function uses per CPU variables, which must not be
>>>> preempted. However, this is fine in the general napi use case where
>>>> the local BH is disabled. But, it's also called in the netlink
>>>> context, which is preemptible. The below patch makes sure that even
>>>> in the netlink path, preemption is disabled.
>>>>
>>>> In addition, the u64_stats_update_begin() sync point was not protected,
>>>> making the sync point part of the per CPU variable fixed this.
>>>>
>>>> Fixes: eac87c413bf9 ("net: openvswitch: reorder masks array based on usage")
>>>> Reported-by: Juri Lelli <jlelli@...hat.com>
>>>> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
>>>> ---
>>>> v2: - Add u64_stats_update_begin() sync point protection
>>>>     - Moved patch to net from net-next branch
>>>>
>>>>  net/openvswitch/flow_table.c |   56 +++++++++++++++++++++++++-----------------
>>>>  net/openvswitch/flow_table.h |    8 +++++-
>>>>  2 files changed, 39 insertions(+), 25 deletions(-)
>>>>
>>>> diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
>>>> index e2235849a57e..d90b4af6f539 100644
>>>> --- a/net/openvswitch/flow_table.c
>>>> +++ b/net/openvswitch/flow_table.c
>>>> @@ -172,7 +172,7 @@ static struct table_instance *table_instance_alloc(int new_size)
>>>>
>>>>  static void __mask_array_destroy(struct mask_array *ma)
>>>>  {
>>>> -    free_percpu(ma->masks_usage_cntr);
>>>> +    free_percpu(ma->masks_usage_stats);
>>>>      kfree(ma);
>>>>  }
>>>>
>>>> @@ -196,15 +196,15 @@ static void tbl_mask_array_reset_counters(struct mask_array *ma)
>>>>          ma->masks_usage_zero_cntr[i] = 0;
>>>>
>>>>          for_each_possible_cpu(cpu) {
>>>> -            u64 *usage_counters = per_cpu_ptr(ma->masks_usage_cntr,
>>>> -                              cpu);
>>>> +            struct mask_array_stats *stats;
>>>>              unsigned int start;
>>>>              u64 counter;
>>>>
>>>> +            stats = per_cpu_ptr(ma->masks_usage_stats, cpu);
>>>>              do {
>>>> -                start = u64_stats_fetch_begin_irq(&ma->syncp);
>>>> -                counter = usage_counters[i];
>>>> -            } while (u64_stats_fetch_retry_irq(&ma->syncp, start));
>>>> +                start = u64_stats_fetch_begin_irq(&stats->syncp);
>>>> +                counter = stats->usage_cntrs[i];
>>>> +            } while (u64_stats_fetch_retry_irq(&stats->syncp, start));
>>>>
>>>>              ma->masks_usage_zero_cntr[i] += counter;
>>>>          }
>>>> @@ -227,9 +227,10 @@ static struct mask_array *tbl_mask_array_alloc(int size)
>>>>                           sizeof(struct sw_flow_mask *) *
>>>>                           size);
>>>>
>>>> -    new->masks_usage_cntr = __alloc_percpu(sizeof(u64) * size,
>>>> -                           __alignof__(u64));
>>>> -    if (!new->masks_usage_cntr) {
>>>> +    new->masks_usage_stats = __alloc_percpu(sizeof(struct mask_array_stats) +
>>>> +                        sizeof(u64) * size,
>>>> +                        __alignof__(u64));
>>>> +    if (!new->masks_usage_stats) {
>>>>          kfree(new);
>>>>          return NULL;
>>>>      }
>>>> @@ -732,7 +733,7 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
>>>>                     u32 *n_cache_hit,
>>>>                     u32 *index)
>>>>  {
>>>> -    u64 *usage_counters = this_cpu_ptr(ma->masks_usage_cntr);
>>>> +    struct mask_array_stats *stats = this_cpu_ptr(ma->masks_usage_stats);
>>>>      struct sw_flow *flow;
>>>>      struct sw_flow_mask *mask;
>>>>      int i;
>>>> @@ -742,9 +743,9 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
>>>>          if (mask) {
>>>>              flow = masked_flow_lookup(ti, key, mask, n_mask_hit);
>>>>              if (flow) {
>>>> -                u64_stats_update_begin(&ma->syncp);
>>>> -                usage_counters[*index]++;
>>>> -                u64_stats_update_end(&ma->syncp);
>>>> +                u64_stats_update_begin(&stats->syncp);
>>>> +                stats->usage_cntrs[*index]++;
>>>> +                u64_stats_update_end(&stats->syncp);
>>>>                  (*n_cache_hit)++;
>>>>                  return flow;
>>>>              }
>>>> @@ -763,9 +764,9 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
>>>>          flow = masked_flow_lookup(ti, key, mask, n_mask_hit);
>>>>          if (flow) { /* Found */
>>>>              *index = i;
>>>> -            u64_stats_update_begin(&ma->syncp);
>>>> -            usage_counters[*index]++;
>>>> -            u64_stats_update_end(&ma->syncp);
>>>> +            u64_stats_update_begin(&stats->syncp);
>>>> +            stats->usage_cntrs[*index]++;
>>>> +            u64_stats_update_end(&stats->syncp);
>>>>              return flow;
>>>>          }
>>>>      }
>>>> @@ -851,9 +852,17 @@ 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);
>>>> +    /* This function gets called trough the netlink interface and therefore
>>>> +     * is preemptible. However, flow_lookup() function needs to be called
>>>> +     * with preemption disabled due to CPU specific variables.
>>>
>>> Is it possible to add some kind of assertion inside flow_lookup() to avoid
>>> this kind of issues in the future?
>>
>> We could do something like WARN_ON_ONCE(preemptible()) but do not think such check should be added to the fast path.
> 
> I meant something like lockdep_assert_preemption_disabled().  This will not
> impact fast path if CONFIG_PROVE_LOCKING disabled, but will allow to catch
> issues during development.

To be clear, I just mean that this could be compiled conditionally under some
debugging config.  Do not know which of existing configs might be used, though. 

> 
>>
>>> It might be also good to update the comment for flow_lookup() function itself.
>>
>> Good idea, will do this in a v3
>>
>>>> +     */
>>>> +    local_bh_disable();
>>>> +    flow = flow_lookup(tbl, ti, ma, key, &n_mask_hit, &n_cache_hit, &index);
>>>> +    local_bh_enable();
>>>> +    return flow;
>>>>  }
>>>>
>>>>  struct sw_flow *ovs_flow_tbl_lookup_exact(struct flow_table *tbl,
>>>> @@ -1109,7 +1118,6 @@ void ovs_flow_masks_rebalance(struct flow_table *table)
>>>>
>>>>      for (i = 0; i < ma->max; i++)  {
>>>>          struct sw_flow_mask *mask;
>>>> -        unsigned int start;
>>>>          int cpu;
>>>>
>>>>          mask = rcu_dereference_ovsl(ma->masks[i]);
>>>> @@ -1120,14 +1128,16 @@ void ovs_flow_masks_rebalance(struct flow_table *table)
>>>>          masks_and_count[i].counter = 0;
>>>>
>>>>          for_each_possible_cpu(cpu) {
>>>> -            u64 *usage_counters = per_cpu_ptr(ma->masks_usage_cntr,
>>>> -                              cpu);
>>>> +            struct mask_array_stats *stats;
>>>> +            unsigned int start;
>>>>              u64 counter;
>>>>
>>>> +            stats = per_cpu_ptr(ma->masks_usage_stats, cpu);
>>>>              do {
>>>> -                start = u64_stats_fetch_begin_irq(&ma->syncp);
>>>> -                counter = usage_counters[i];
>>>> -            } while (u64_stats_fetch_retry_irq(&ma->syncp, start));
>>>> +                start = u64_stats_fetch_begin_irq(&stats->syncp);
>>>> +                counter = stats->usage_cntrs[i];
>>>> +            } while (u64_stats_fetch_retry_irq(&stats->syncp,
>>>> +                               start));
>>>>
>>>>              masks_and_count[i].counter += counter;
>>>>          }
>>>> diff --git a/net/openvswitch/flow_table.h b/net/openvswitch/flow_table.h
>>>> index 6e7d4ac59353..43144396e192 100644
>>>> --- a/net/openvswitch/flow_table.h
>>>> +++ b/net/openvswitch/flow_table.h
>>>> @@ -38,12 +38,16 @@ struct mask_count {
>>>>      u64 counter;
>>>>  };
>>>>
>>>> +struct mask_array_stats {
>>>> +    struct u64_stats_sync syncp;
>>>> +    u64 usage_cntrs[];
>>>> +};
>>>> +
>>>>  struct mask_array {
>>>>      struct rcu_head rcu;
>>>>      int count, max;
>>>> -    u64 __percpu *masks_usage_cntr;
>>>> +    struct mask_array_stats __percpu *masks_usage_stats;
>>>>      u64 *masks_usage_zero_cntr;
>>>> -    struct u64_stats_sync syncp;
>>>>      struct sw_flow_mask __rcu *masks[];
>>>>  };
>>>>
>>>>
>>>> _______________________________________________
>>>> dev mailing list
>>>> dev@...nvswitch.org
>>>> https://mail.openvswitch.org/mailman/listinfo/ovs-dev
>>>>
>>
> 

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