Re: [PATCH v7 5/6] lib/dlock-list: Enable faster lookup with hashing

[Davidlohr Bueso <dave@stgolabs.net>]

On Thu, 05 Oct 2017, Waiman Long wrote:

>Insertion and deletion is relatively cheap and mostly contention
>free for dlock-list. Lookup, on the other hand, can be rather costly
>because all the lists in a dlock-list will have to be iterated.
>
>Currently dlock-list insertion is based on the cpu that the task is
>running on. So a given object can be inserted into any one of the
>lists depending on what the current cpu is.
>
>This patch provides an alternative way of list selection. The caller
>can provide a object context which will be hashed to one of the list
>in a dlock-list. The object can then be added into that particular
>list. Lookup can be done by iterating elements in the provided list
>only instead of all the lists in a dlock-list.

Unless I'm misusing the api, I could not find a standard way of
iterating a _particular_ list head (the one the dlock_list_hash()
returned). This is because iterators always want the all the heads.

Also, in my particular epoll case I'd need the head->lock _not_ to
be dropped after the iteration, and therefore is pretty adhoc.
Currently we do:

dlist_for_each_entry() {
	// acquire head->lock for each list
}
// no locks held
dlist_add()

I'm thinking perhaps we could have dlist_check_add() which passes a
callback to ensure we want to add the node. The function could acquire
the head->lock and not release it until the very end.

Thanks,
Davidlohr