Re: [PATCH v3] netfilter: nft_hash: bug fixes and resizing

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Tue, Mar 04, 2014 at 04:21:51PM +0100, Patrick McHardy wrote:
> The hash set type is very broken and was never meant to be merged in this
> state. Missing RCU synchronization on element removal, leaking chain
> refcounts when used as a verdict map, races during lookups, a fixed table
> size are probably just some of the problems. Luckily it is currently
> never chosen by the kernel when the rbtree type is also available.
> 
> Rewrite it to be usable.
> 
> The new implementation supports automatic hash table resizing using RCU,
> based on Paul McKenney's and Josh Triplett's algorithm "Optimized Resizing
> For RCU-Protected Hash Tables" described in [1].
> 
> Resizing doesn't require a second list head in the elements, it works by
> chosing a hash function that remaps elements to a predictable set of buckets,
> only resizing by integral factors and
> 
> - during expansion: linking new buckets to the old bucket that contains
>   elements for any of the new buckets, thereby creating imprecise chains,
>   then incrementally seperating the elements until the new buckets only
>   contain elements that hash directly to them.
> 
> - during shrinking: linking the hash chains of all old buckets that hash
>   to the same new bucket to form a single chain.
> 
> Expansion requires at most the number of elements in the longest hash chain
> grace periods, shrinking requires a single grace period.
> 
> Due to the requirement of having hash chains/elements linked to multiple
> buckets during resizing, homemade single linked lists are used instead of
> the existing list helpers, that don't support this in a clean fashion.
> As a side effect, the amount of memory required per element is reduced by
> one pointer.
> 
> Expansion is triggered when the load factors exceeds 75%, shrinking when
> the load factor goes below 30%. Both operations are allowed to fail and
> will be retried on the next insertion or removal if their respective
> conditions still hold.

Applied, thanks Patrick.