Re: [PATCH 6/8] rhashtable: further improve stability of rhashtable_walk

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

On Fri, May 04, 2018 at 01:54:14PM +1000, 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 (certainly not so easy) to achieve this 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.
>  No current user of rhltable_walk_enter() calls
>  rhashtable_walk_start() more than once, so no current code
>  could benefit from a more reliable walk of rhltables.
> 
>  This patch only attempts to improve walks for rhashtables.
>  - 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>

I'm a bit torn on this.  On the hand this is definitely an improvement
over the status quo.  On the other this does not work on rhltable and
we do have a way of fixing it for both rhashtable and rhltable.

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