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Date:   Sun, 30 Aug 2020 23:02:19 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     trix@...hat.com
Cc:     pshelar@....org, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        netdev <netdev@...r.kernel.org>, dev@...nvswitch.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: openvswitch: pass NULL for unused parameters

On Sun, Aug 30, 2020 at 6:17 PM <trix@...hat.com> wrote:
>
> From: Tom Rix <trix@...hat.com>
>
> clang static analysis flags these problems
>
> flow_table.c:713:2: warning: The expression is an uninitialized
>   value. The computed value will also be garbage
>         (*n_mask_hit)++;
>         ^~~~~~~~~~~~~~~
> flow_table.c:748:5: warning: The expression is an uninitialized
>   value. The computed value will also be garbage
>                                 (*n_cache_hit)++;
>                                 ^~~~~~~~~~~~~~~~
>
> These are not problems because neither pararmeter is used

parameter

> by the calling function.
>
> Looking at all of the calling functions, there are many
> cases where the results are unused.  Passing unused
> parameters is a waste.
>
> To avoid passing unused parameters, rework the
> masked_flow_lookup() and flow_lookup() routines to check
> for NULL parameters and change the unused parameters to NULL.
>
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  net/openvswitch/flow_table.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
>
> diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
> index e2235849a57e..18e7fa3aa67e 100644
> --- a/net/openvswitch/flow_table.c
> +++ b/net/openvswitch/flow_table.c
> @@ -710,7 +710,8 @@ static struct sw_flow *masked_flow_lookup(struct table_instance *ti,
>         ovs_flow_mask_key(&masked_key, unmasked, false, mask);
>         hash = flow_hash(&masked_key, &mask->range);
>         head = find_bucket(ti, hash);
> -       (*n_mask_hit)++;
> +       if (n_mask_hit)
> +               (*n_mask_hit)++;
>
>         hlist_for_each_entry_rcu(flow, head, flow_table.node[ti->node_ver],
>                                 lockdep_ovsl_is_held()) {
> @@ -745,7 +746,8 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
>                                 u64_stats_update_begin(&ma->syncp);
>                                 usage_counters[*index]++;
>                                 u64_stats_update_end(&ma->syncp);
> -                               (*n_cache_hit)++;
> +                               if (n_cache_hit)
> +                                       (*n_cache_hit)++;
>                                 return flow;
>                         }
>                 }
> @@ -798,9 +800,8 @@ struct sw_flow *ovs_flow_tbl_lookup_stats(struct flow_table *tbl,
>         *n_cache_hit = 0;

>         if (unlikely(!skb_hash || mc->cache_size == 0)) {
>                 u32 mask_index = 0;
> -               u32 cache = 0;
>
> -               return flow_lookup(tbl, ti, ma, key, n_mask_hit, &cache,
> +               return flow_lookup(tbl, ti, ma, key, n_mask_hit, NULL,
>                                    &mask_index);

Can it be done for mask_index as well?

>         }
>
> @@ -849,11 +850,9 @@ struct sw_flow *ovs_flow_tbl_lookup(struct flow_table *tbl,
>  {
>         struct table_instance *ti = rcu_dereference_ovsl(tbl->ti);
>         struct mask_array *ma = rcu_dereference_ovsl(tbl->mask_array);
> -       u32 __always_unused n_mask_hit;
> -       u32 __always_unused n_cache_hit;
>         u32 index = 0;
>

> -       return flow_lookup(tbl, ti, ma, key, &n_mask_hit, &n_cache_hit, &index);
> +       return flow_lookup(tbl, ti, ma, key, NULL, NULL, &index);

Ditto.

>  }
>
>  struct sw_flow *ovs_flow_tbl_lookup_exact(struct flow_table *tbl,
> @@ -865,7 +864,6 @@ struct sw_flow *ovs_flow_tbl_lookup_exact(struct flow_table *tbl,
>         /* Always called under ovs-mutex. */
>         for (i = 0; i < ma->max; i++) {
>                 struct table_instance *ti = rcu_dereference_ovsl(tbl->ti);
> -               u32 __always_unused n_mask_hit;
>                 struct sw_flow_mask *mask;
>                 struct sw_flow *flow;
>
> @@ -873,7 +871,7 @@ struct sw_flow *ovs_flow_tbl_lookup_exact(struct flow_table *tbl,
>                 if (!mask)
>                         continue;
>
> -               flow = masked_flow_lookup(ti, match->key, mask, &n_mask_hit);
> +               flow = masked_flow_lookup(ti, match->key, mask, NULL);
>                 if (flow && ovs_identifier_is_key(&flow->id) &&
>                     ovs_flow_cmp_unmasked_key(flow, match)) {
>                         return flow;
> --
> 2.18.1
>


-- 
With Best Regards,
Andy Shevchenko

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