From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Fastabend Subject: Re: nft_hash rhashtable question Date: Mon, 12 Jan 2015 14:53:51 -0800 Message-ID: <54B4507F.1020605@gmail.com> References: <54B44B10.7000009@gmail.com> <20150112224249.GA11857@casper.infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: netdev To: tgraf Return-path: Received: from mail-ob0-f178.google.com ([209.85.214.178]:47880 "EHLO mail-ob0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751388AbbALWyG (ORCPT ); Mon, 12 Jan 2015 17:54:06 -0500 Received: by mail-ob0-f178.google.com with SMTP id gq1so24841763obb.9 for ; Mon, 12 Jan 2015 14:54:06 -0800 (PST) In-Reply-To: <20150112224249.GA11857@casper.infradead.org> Sender: netdev-owner@vger.kernel.org List-ID: 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