Subject: Re: nft_hash rhashtable question

From: John Fastabend <[email-masked]>
Date: Mon, 12 Jan 2015 14:53:51 -0800

On 01/12/2015 02:42 PM, tgraf wrote:
> On 01/12/15 at 02:30pm, John Fastabend wrote:
>>>         /* Stop an eventual async resizing */
>>>         priv->being_destroyed = true;
>
> This aborts and eventual resize in the background.
>
>>>         mutex_lock(&priv->mutex); <-- get the lock so we have single updater
>
> this ensures that the resize finished.
>
>>>         tbl = rht_dereference(priv->tbl, priv);
>>>         for (i = 0; i < tbl->size; i++) {
>>>                 rht_for_each_entry_safe(he, pos, next, tbl, i, node)
>>>                         nft_hash_elem_destroy(set, he);              <-- does a kfree on he?
>>>         }
>>>         mutex_unlock(&priv->mutex); <-- release the lock
>>>
>>>         rhashtable_destroy(priv);
>
> Since no insert or removal can occur we can be assured that no new
> entry was added in the meantime so we can destroy the rhashtable
> without any further protection.
>
>>> }
>>
>>
>> Is it really safe to kfree 'he' without waiting a grace
>> period for any rcu readers to drop the reference?
>>
>> I'm considering what happens if nft_hash_destroy runs in
>> parallel with nft_hash_lookup?
>
> The nft_set API ensures that a destroy can't occur in parallel to
> an insertion or removal. All we have to ensure is that any resizing
> in the background is aborted and completed.
>

It must also somehow ensure there are no readers with a reference
as well. Thanks for the explanation.

> If you look at the code before the rhashtable there was no locking
> at all:
>
> static void nft_hash_destroy(const struct nft_set *set)
> {
>          const struct nft_hash *priv = nft_set_priv(set);
>          const struct nft_hash_table *tbl = nft_dereference(priv->tbl);
>          struct nft_hash_elem *he, *next;
>          unsigned int i;
>
>          for (i = 0; i < tbl->size; i++) {
>                  for (he = nft_dereference(tbl->buckets[i]); he != NULL;
>                       he = next) {
>                          next = nft_dereference(he->next);
>                          nft_hash_elem_destroy(set, he);
>                  }
>          }
>          kfree(tbl);
> }
>

-- 
John Fastabend         Intel Corporation