Re: [PATCH net-next] rhashtable: further improve stability of rhashtable_walk

[Herbert Xu <herbert@gondor.apana.org.au>]

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