netdev - Re: [PATCH net-next] bpf: Optimize lpm trie delete

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Message-ID: <CAEfhGiy_hC0tCd=5qoPZ1okyMCYsgMQbKAa0ms1Tb9cnYDaLBg@mail.gmail.com>
Date:   Wed, 20 Sep 2017 14:51:56 -0400
From:   Craig Gallek <kraigatgoog@...il.com>
To:     Daniel Mack <daniel@...que.org>
Cc:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] bpf: Optimize lpm trie delete

On Wed, Sep 20, 2017 at 12:51 PM, Daniel Mack <daniel@...que.org> wrote:
> Hi Craig,
>
> Thanks, this looks much cleaner already :)
>
> On 09/20/2017 06:22 PM, Craig Gallek wrote:
>> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
>> index 9d58a576b2ae..b5a7d70ec8b5 100644
>> --- a/kernel/bpf/lpm_trie.c
>> +++ b/kernel/bpf/lpm_trie.c
>> @@ -397,7 +397,7 @@ static int trie_delete_elem(struct bpf_map *map, void *_key)
>>       struct lpm_trie_node __rcu **trim;
>>       struct lpm_trie_node *node;
>>       unsigned long irq_flags;
>> -     unsigned int next_bit;
>> +     unsigned int next_bit = 0;
>
> This default assignment seems wrong, and I guess you only added it to
> squelch a compiler warning?
Yes, this variable is only initialized after the lookup iterations
below (meaning it will never be initialized the the root-removal
case).

> [...]
>
>> +     /* If the node has one child, we may be able to collapse the tree
>> +      * while removing this node if the node's child is in the same
>> +      * 'next bit' slot as this node was in its parent or if the node
>> +      * itself is the root.
>> +      */
>> +     if (trim == &trie->root) {
>> +             next_bit = node->child[0] ? 0 : 1;
>> +             rcu_assign_pointer(trie->root, node->child[next_bit]);
>> +             kfree_rcu(node, rcu);
>
> I don't think you should treat this 'root' case special.
>
> Instead, move the 'next_bit' assignment outside of the condition ...
I'm not quite sure I follow.  Are you saying do something like this:

if (trim == &trie->root) {
  next_bit = node->child[0] ? 0 : 1;
}
if (rcu_access_pointer(node->child[next_bit])) {
...

This would save a couple lines of code, but I think the as-is
implementation is slightly easier to understand.  I don't have a
strong opinion either way, though.

Thanks for the pointers,
Craig