rhashtable: Kill harmless RCU warning in rhashtable_walk_init

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: rhashtable: Kill harmless RCU warning in rhashtable_walk_init
Date: Fri, 18 Dec 2015 14:24:50 +0800	[thread overview]
Message-ID: <20151218062450.GA19749@gondor.apana.org.au> (raw)
In-Reply-To: <20151218053416.GA19479@gondor.apana.org.au>

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 <ying.huang@linux.intel.com>
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
