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SubjectRe: [PATCH net-next] rhashtable: further improve stability of rhashtable_walk
On Wed, Dec 05, 2018 at 02:51:02PM +1100, NeilBrown wrote:
>
> If the sequence:
> obj = rhashtable_walk_next(iter);
> rhashtable_walk_stop(iter);
> rhashtable_remove_fast(ht, &obj->head, params);
> rhashtable_walk_start(iter);
>
> races with another thread inserting or removing
> an object on the same hash chain, a subsequent
> rhashtable_walk_next() is not guaranteed to get the "next"
> object. It is possible that an object could be
> repeated, or missed.
>
> This can be made more reliable by keeping the objects in a hash chain
> sorted by memory address. A subsequent rhashtable_walk_next()
> call can reliably find the correct position in the list, and thus
> find the 'next' object.
>
> It is not possible to take this approach with an rhltable as keeping
> the hash chain in order is not so easy. When the first object with a
> given key is removed, it is replaced in the chain with the next
> object with the same key, and the address of that object may not be
> correctly ordered.
> I have not yet found any way to achieve the same stability
> with rhltables, that doesn't have a major impact on lookup
> or insert. No code currently in Linux would benefit from
> such extra stability.
>
> With this patch:
> - a new object is always inserted after the last object with a
> smaller address, or at the start.
> - when rhashtable_walk_start() is called, it records that 'p' is not
> 'safe', meaning that it cannot be dereferenced. The revalidation
> that was previously done here is moved to rhashtable_walk_next()
> - when rhashtable_walk_next() is called while p is not NULL and not
> safe, it walks the chain looking for the first object with an
> address greater than p and returns that. If there is none, it moves
> to the next hash chain.
>
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
>
> This is a resend of a patch that I sent back in July. I couldn't
> applied then because it assumed another rhashtable patch which hadn't
> landed yet - it now has.

I thought we had agreed to drop this because nobody needs it
currently and it doesn't handle rhlist?

Cheers,
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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