From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D328AC04EB9 for ; Mon, 3 Dec 2018 23:32:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8693220851 for ; Mon, 3 Dec 2018 23:32:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8693220851 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=davemloft.net Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725971AbeLCXcL (ORCPT ); Mon, 3 Dec 2018 18:32:11 -0500 Received: from shards.monkeyblade.net ([23.128.96.9]:49838 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725903AbeLCXcL (ORCPT ); Mon, 3 Dec 2018 18:32:11 -0500 Received: from localhost (unknown [IPv6:2601:601:9f80:35cd::bf5]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) (Authenticated sender: davem-davemloft) by shards.monkeyblade.net (Postfix) with ESMTPSA id 3C330133E98E5; Mon, 3 Dec 2018 15:32:10 -0800 (PST) Date: Mon, 03 Dec 2018 15:32:09 -0800 (PST) Message-Id: <20181203.153209.2147928207639502351.davem@davemloft.net> To: neilb@suse.com Cc: herbert@gondor.apana.org.au, tgraf@suug.ch, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, eric.dumazet@gmail.com Subject: Re: [PATCH v3] rhashtable: detect when object movement between tables might have invalidated a lookup From: David Miller In-Reply-To: <87pnuno479.fsf@notabene.neil.brown.name> References: <20181116055551.zxfydbpoicslsfkg@gondor.apana.org.au> <878t1tece0.fsf@notabene.neil.brown.name> <87pnuno479.fsf@notabene.neil.brown.name> X-Mailer: Mew version 6.8 on Emacs 26.1 Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.12 (shards.monkeyblade.net [149.20.54.216]); Mon, 03 Dec 2018 15:32:10 -0800 (PST) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: NeilBrown Date: Fri, 30 Nov 2018 10:26:50 +1100 > > Some users of rhashtables might need to move an object from one table > to another - this appears to be the reason for the incomplete usage > of NULLS markers. > > To support these, we store a unique NULLS_MARKER at the end of > each chain, and when a search fails to find a match, we check > if the NULLS marker found was the expected one. If not, the search > may not have examined all objects in the target bucket, so it is > repeated. > > The unique NULLS_MARKER is derived from the address of the > head of the chain. As this cannot be derived at load-time the > static rhnull in rht_bucket_nested() needs to be initialised > at run time. > > Any caller of a lookup function must still be prepared for the > possibility that the object returned is in a different table - it > might have been there for some time. > > Note that this does NOT provide support for other uses of > NULLS_MARKERs such as allocating with SLAB_TYPESAFE_BY_RCU or changing > the key of an object and re-inserting it in the same table. > These could only be done safely if new objects were inserted > at the *start* of a hash chain, and that is not currently the case. > > Signed-off-by: NeilBrown Applied to net-next.