From: Herbert Xu <herbert@gondor.apana.org.au> To: kernel test robot <ying.huang@linux.intel.com> Cc: lkp@01.org, LKML <linux-kernel@vger.kernel.org>, Colin Ian King <colin.king@canonical.com>, 0day robot <fengguang.wu@intel.com>, netdev@vger.kernel.org Subject: Re: [lkp] [rhashtable] f9f51b8070: INFO: suspicious RCU usage. ] Date: Fri, 18 Dec 2015 13:34:16 +0800 [thread overview] Message-ID: <20151218053416.GA19479@gondor.apana.org.au> (raw) In-Reply-To: <874mfgedrp.fsf@yhuang-dev.intel.com> 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. ---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. Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au> 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); list_add(&iter->walker->list, &iter->walker->tbl->walkers); spin_unlock(&ht->lock); -- Email: Herbert Xu <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
WARNING: multiple messages have this Message-ID (diff)
From: Herbert Xu <herbert@gondor.apana.org.au> To: lkp@lists.01.org Subject: Re: [rhashtable] f9f51b8070: INFO: suspicious RCU usage. ] Date: Fri, 18 Dec 2015 13:34:16 +0800 [thread overview] Message-ID: <20151218053416.GA19479@gondor.apana.org.au> (raw) In-Reply-To: <874mfgedrp.fsf@yhuang-dev.intel.com> [-- Attachment #1: Type: text/plain, Size: 2351 bytes --] 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. ---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. Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au> 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); list_add(&iter->walker->list, &iter->walker->tbl->walkers); spin_unlock(&ht->lock); -- Email: Herbert Xu <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
next prev parent reply other threads:[~2015-12-18 5:34 UTC|newest] Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-12-18 1:39 [lkp] [rhashtable] f9f51b8070: INFO: suspicious RCU usage. ] kernel test robot 2015-12-18 1:39 ` kernel test robot 2015-12-18 5:34 ` Herbert Xu [this message] 2015-12-18 5:34 ` Herbert Xu 2015-12-18 6:24 ` rhashtable: Kill harmless RCU warning in rhashtable_walk_init Herbert Xu 2015-12-18 6:24 ` Herbert Xu 2015-12-18 12:54 ` Eric Dumazet 2015-12-18 12:54 ` Eric Dumazet 2015-12-18 13:14 ` Herbert Xu 2015-12-18 13:14 ` Herbert Xu 2015-12-18 21:27 ` David Miller 2015-12-18 21:27 ` David Miller 2015-12-19 2:45 ` [PATCH v2] " Herbert Xu 2015-12-19 2:45 ` Herbert Xu 2015-12-19 4:42 ` David Miller 2015-12-19 4:42 ` David Miller 2015-12-19 4:47 ` [LKP] " Fengguang Wu 2015-12-19 4:47 ` Fengguang Wu
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