Re: [RFC] lockless radix tree readside

[Nick Piggin]

Joe Seigh wrote:

> Well, I don't have a kernel development set up so I can't comment on
> the specific patch but I have done some minor experimentation with reader
> lock-free b-trees, specifically insert, delete, and rotate (no actual
> balancing heuristics though) so I can comment on what some of the 
> general issues are.
>
> You need to have a serialization point in your tree modifications so
> the change becomes atomically visible to threads reading the tree.

Yes, that is the memory barrier in rcu_assign_pointer.

> This is important for the semantics of your data structure.  It's not
> good to have a node become temporarily invisible to readers if the
> tree operation involved moving a node or subtree around with more than
> a single link modification.  So you will likely find yourself needing to 
> use
> COW (copy on write) or PCOW (partial copy on write), particularly on
> deletes of non leaf nodes. PCOW is naturally better, especially if you
> can minimize the number of nodes that have to be copied.

Fortunately the radix tree never needs to do anything like this.
It doesn't move nodes or subtrees - the only modification operations
needed are to insert and delete items (ignoring the tag operations,
which are done under lock).

> So that's probably what you want to have in your documentation; what
> the serialization points are, your COW or PCOW mechanism, and how
> they preserve semantics.
>
> Also I assume you're returning lookups by value and not reference
> unless they're refcounted (which naturally since you're using RCU
> can be incremented safely if the refcount is not zero)

It can return either. It is up to the reader to do the right thing
in either case (which will need a note in the API comments).

Thanks,
Nick

--
SUSE Labs, Novell Inc.

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