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=-2.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT 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 8D3EEECDFB1 for ; Mon, 16 Jul 2018 02:17:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DB54E208E1 for ; Mon, 16 Jul 2018 02:17:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DB54E208E1 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=gondor.apana.org.au 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 S1727653AbeGPCmJ (ORCPT ); Sun, 15 Jul 2018 22:42:09 -0400 Received: from orcrist.hmeau.com ([104.223.48.154]:34542 "EHLO deadmen.hmeau.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727296AbeGPCmI (ORCPT ); Sun, 15 Jul 2018 22:42:08 -0400 Received: from gondobar.mordor.me.apana.org.au ([192.168.128.4] helo=gondobar) by deadmen.hmeau.com with esmtps (Exim 4.89 #2 (Debian)) id 1fet4M-0003I9-Ol; Mon, 16 Jul 2018 10:16:58 +0800 Received: from herbert by gondobar with local (Exim 4.89) (envelope-from ) id 1fet4H-0001ux-Mp; Mon, 16 Jul 2018 10:16:53 +0800 Date: Mon, 16 Jul 2018 10:16:53 +0800 From: Herbert Xu To: NeilBrown Cc: David Miller , tgraf@suug.ch, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, eric.dumazet@gmail.com Subject: Re: [PATCH - revised] rhashtable: detect when object movement might have invalidated a lookup Message-ID: <20180716021653.xtbyetdfc2uzxub3@gondor.apana.org.au> References: <20180601160613.7ud25g2ux55k3bma@gondor.apana.org.au> <87k1q8yh70.fsf@notabene.neil.brown.name> <20180711.224658.2077863065492745521.davem@davemloft.net> <20180711.224801.1129067473269289703.davem@davemloft.net> <87fu0kt5m0.fsf@notabene.neil.brown.name> <20180716005134.q4rfkfn3m5zzwt62@gondor.apana.org.au> <874lh0t1ls.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <874lh0t1ls.fsf@notabene.neil.brown.name> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 16, 2018 at 11:23:43AM +1000, NeilBrown wrote: > > kmem_cache_free() directly. For this, I need rhashtable to be safe if > an object is deleted and immediately re-inserted into the same hash > chain. This means that rcu_read_lock(); A = rhashtable_lookup(); use(A); rcu_read_unlock(); A can turn into object B when it is used. That is just too strange for words. Can we see some actual code on how this works? For comparison, the existing net code where this happens A doesn't actually change and it simply moves from one hashtable to another. I'm relucant to add semantics that would restrain on how rhashtable works unless we have real and valid use-cases for it. Cheers, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt