Re: [PATCH 1/3] rhashtable: further improve stability of rhashtable_walk

[Paolo Abeni]

On Fri, 2018-07-06 at 17:11 +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.

The above scenario is very similar to the one I'm running:

   rhashtable_walk_next(iter);
   rhashtable_walk_stop(iter);     
   // rhashtable change not yet identified, could be either
   // remove, insert or even rehash
   rhashtable_walk_start(iter);
   rhashtable_walk_next(iter);

but I'm seeing use-after-free there. I'll try this patch to see if
solves my issue.

Note: the code under test is a pending new patch I'm holding due to the
above issue, I can send it as RFC to share the code if you think it may
help.

> @@ -867,15 +866,39 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
>  	bool rhlist = ht->rhlist;
>  
>  	if (p) {
> -		if (!rhlist || !(list = rcu_dereference(list->next))) {
> -			p = rcu_dereference(p->next);
> -			list = container_of(p, struct rhlist_head, rhead);
> -		}
> -		if (!rht_is_a_nulls(p)) {
> -			iter->skip++;
> -			iter->p = p;
> -			iter->list = list;
> -			return rht_obj(ht, rhlist ? &list->rhead : p);
> +		if (!rhlist && iter->p_is_unsafe) {
> +			/*
> +			 * First time next() was called after start().
> +			 * Need to find location of 'p' in the list.
> +			 */
> +			struct rhash_head *p;
> +
> +			iter->skip = 0;
> +			rht_for_each_rcu(p, iter->walker.tbl, iter->slot) {
> +				iter->skip++;
> +				if (p <= iter->p)
> +					continue;

Out of sheer ignorance, I really don't understand the goal of the above
conditional ?!?

Should it possibly be something like:
				if (p != iter->p->next)

instead? 
But I think we can't safely dereference 'p' yet ?!?

I'm sorry for the possibly dumb comments, rhashtable internals are
somewhat obscure to me, but I'm really interested in this topic.

Cheers,

Paolo