Re: [ovs-dev] [PATCH net v2] net: openvswitch: fix to make sure flow_lookup() is not preempted

[Ilya Maximets <i.maximets@ovn.org>]

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@redhat.com>
> Signed-off-by: Eelco Chaudron <echaudro@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?

It might be also good to update the comment for flow_lookup() function itself.

> +	 */
> +	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[];
>  };
>  
> 
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