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

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Message-ID: <dc09f044-be69-a2a9-bf77-331ce558eee9@zonque.org>
Date:   Thu, 21 Sep 2017 00:56:19 +0200
From:   Daniel Mack <daniel@...que.org>
To:     Craig Gallek <kraigatgoog@...il.com>
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 09/20/2017 08:51 PM, Craig Gallek wrote:
> 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).

Right, and once set, it's only updated in case we don't have an exact
match and try to drill down further.

>> [...]
>>
>>> +     /* 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.

Me neither :)

My idea was to set

  next_bit = node->child[0] ? 0 : 1;

unconditionally, because it should result in the same in both cases.

It might be a bit of bike shedding, but I dislike this default
assignment, and I believe that not relying on next_bit to be set as a
side effect of the lookup loop makes the code a bit more readable.

WDYT?


Thanks,
Daniel