Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753365AbbLRMyU (ORCPT ); Fri, 18 Dec 2015 07:54:20 -0500 Received: from mail-pf0-f179.google.com ([209.85.192.179]:35704 "EHLO mail-pf0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750917AbbLRMyQ (ORCPT ); Fri, 18 Dec 2015 07:54:16 -0500 Message-ID: <1450443254.8474.120.camel@edumazet-glaptop2.roam.corp.google.com> Subject: Re: rhashtable: Kill harmless RCU warning in rhashtable_walk_init From: Eric Dumazet To: Herbert Xu Cc: kernel test robot , lkp@01.org, LKML , Colin Ian King , 0day robot , netdev@vger.kernel.org Date: Fri, 18 Dec 2015 04:54:14 -0800 In-Reply-To: <20151218062450.GA19749@gondor.apana.org.au> References: <874mfgedrp.fsf@yhuang-dev.intel.com> <20151218053416.GA19479@gondor.apana.org.au> <20151218062450.GA19749@gondor.apana.org.au> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4-0ubuntu2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3060 Lines: 74 On Fri, 2015-12-18 at 14:24 +0800, Herbert Xu wrote: > On Fri, Dec 18, 2015 at 01:34:16PM +0800, Herbert Xu wrote: > > On Fri, Dec 18, 2015 at 09:39:22AM +0800, kernel test robot wrote: > > > FYI, we noticed the below changes on > > > > > > https://github.com/0day-ci/linux Herbert-Xu/rhashtable-Fix-walker-list-corruption/20151216-164833 > > > commit f9f51b8070be3e829100614a7372b219723b864f ("rhashtable: Fix walker list corruption") > > > > > > [ 8.933376] =============================== > > > [ 8.933376] =============================== > > > [ 8.934629] [ INFO: suspicious RCU usage. ] > > > [ 8.934629] [ INFO: suspicious RCU usage. ] > > > [ 8.935941] 4.4.0-rc3-00995-gf9f51b8 #2 Not tainted > > > [ 8.935941] 4.4.0-rc3-00995-gf9f51b8 #2 Not tainted > > > [ 8.937494] ------------------------------- > > > [ 8.937494] ------------------------------- > > > [ 8.938818] lib/rhashtable.c:504 suspicious rcu_dereference_protected() usage! > > > [ 8.938818] lib/rhashtable.c:504 suspicious rcu_dereference_protected() usage! > > > > This is actually a false positive because the new spin lock that > > we hold prevents ht->tbl from disappearing under us. So here is > > a patch to kill the warning with a comment. > > Resent with a proper patch subject and reported-by. > > ---8<--- > The commit f9f51b8070be3e829100614a7372b219723b864f ("rhashtable: > Fix walker list corruption") causes a suspicious RCU usage warning > because we no longer hold ht->mutex when we dereference ht->tbl. > > However, this is a false positive because we now hold ht->lock > which also guarantees that ht->tbl won't disppear from under us. > > This patch kills the warning by using rcu_dereference_raw and > adding a comment. > > Reported-by: kernel test robot > Signed-off-by: Herbert Xu > > diff --git a/lib/rhashtable.c b/lib/rhashtable.c > index eb9240c..3404b06 100644 > --- a/lib/rhashtable.c > +++ b/lib/rhashtable.c > @@ -519,7 +519,11 @@ int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter) > return -ENOMEM; > > spin_lock(&ht->lock); > - iter->walker->tbl = rht_dereference(ht->tbl, ht); > + /* We do not need RCU protection because we hold ht->lock > + * which guarantees that if we see ht->tbl then it won't > + * die on us. > + */ > + iter->walker->tbl = rcu_dereference_raw(ht->tbl); You can avoid the comment by using the self documented and lockdep enabled primitive iter->walker->tbl = rcu_dereference_protected(ht->tbl, lockdep_is_held(&ht->lock)); But, storing the ht->tbl and then releasing the lock immediately after escapes RCU protection. So why do we store ht->tbl in the first place ? What exactly prevents it from disappearing after lock is released ? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/