On 09/20/2017 08:51 PM, Craig Gallek wrote:
> On Wed, Sep 20, 2017 at 12:51 PM, Daniel Mack <dan...@zonque.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
