Re: [B.A.T.M.A.N.] The current state of the batman-adv vis code

[Matthias Schiffer <mschiffer@universe-factory.net>]

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On 05/08/2012 07:59 AM, Marek Lindner wrote:
> On Tuesday, May 08, 2012 03:35:07 Matthias Schiffer wrote:

>> overall hash consistency might be a issue though - I would propose adding a
>> hash_update function that updates a hash entry without deleting and
>> re-adding the hlist node.
> 
> Can you give an example of such an "update" ? Where would we need it ?

It would be useful to keep the output consistent even when the hash is
updated while the output is generated - as a delete-add sequence adds
the new element at the head of the hlist, a RCU-locked reader will not
see the element when the traversal position is between the head and the
old position of the element. An hash_update could use
hlist_replace_rcu() to replace an element in a way that each reader
either sees the old or the new version, but none loses it completely.

A hash_update_if version that gets an additional callback that is
provided with the old and the new element and gets to decide which
element to keep in the hash could be used to compare the sequence
numbers in the vis code and update the hash atomically.

Matthias


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