netdev - Re: [PATCH net-next] net: openvswitch: fix to make sure flow

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Date:   Tue, 13 Oct 2020 14:53:07 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Eelco Chaudron <echaudro@...hat.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, dev@...nvswitch.org,
        kuba@...nel.org, pabeni@...hat.com, pshelar@....org,
        jlelli@...hat.com
Subject: Re: [PATCH net-next] net: openvswitch: fix to make sure
 flow_lookup() is not preempted

On 2020-10-13 14:44:19 [+0200], 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.
> 
> 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>
> ---
>  net/openvswitch/flow_table.c |   10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
> index 87c286ad660e..16289386632b 100644
> --- a/net/openvswitch/flow_table.c
> +++ b/net/openvswitch/flow_table.c
> @@ -850,9 +850,17 @@ 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.
> +	 */

Once again. u64_stats_update_begin(). What protects you against
concurrent access.

> +	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,
> 

Sebastian