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

[Paolo Abeni]

On Fri, 2018-07-06 at 19:55 +1000, NeilBrown wrote:
> On Fri, Jul 06 2018, Paolo Abeni wrote:
> 
> > 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.
> 
> I'd suggest post it.  I may not get a chance to look at it, but if you
> don't post it, then I definitely won't :-)

Oks, thanks, I just spammed the list (and you ;)

> > > @@ -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 ?!?
> 
> I hoped the patch description would cover that:
>      With this patch:
>      - a new object is always inserted after the last object with a
>        smaller address, or at the start.  This preserves the property,
>        important when allowing objects to be removed and re-added, that
>        an object is never inserted *after* a position that it previously
>        held in the list.
> 
> The items in each table slot are stored in order of the address of the
> item.  So to find the first item in a slot that was not before the
> previously returned item (iter->p), we step forward while this item is
> <= that one. 
> 
> Does that help at all?

Yes, it's very clear. Before I dumbly skipped some slices of the patch.

Thanks,

Paolo