[ovs-dev] [PATCH net v2] net: openvswitch: fix to make sure flow_lookup() is not preempted
Eelco Chaudron
echaudro at redhat.com
Thu Oct 15 12:42:52 UTC 2020
On 15 Oct 2020, at 13:22, Ilya Maximets wrote:
> 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 at redhat.com>
>>>>> Signed-off-by: Eelco Chaudron <echaudro at redhat.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.
Looked at the debug flags, but there is not really one that makes sense
to use. Also using a general one like DEBUG_MISC will probably no help,
as none would probably run OVS tests with this flag enabled. So for now
I would leave it as is unless someone has a better idea…
>>>
>>>> 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 at openvswitch.org
>>>>> https://mail.openvswitch.org/mailman/listinfo/ovs-dev
>>>>>
>>>
>>
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