linux-kernel - [ 16/27] net: force a reload of first item in hlist_nulls_for_each_entry

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Message-Id: <20130625183519.559502143@linuxfoundation.org>
Date:	Tue, 25 Jun 2013 11:35:35 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Boris Zhmurov <zhmurov@...dex-team.ru>,
	Roman Gushchin <klamm@...dex-team.ru>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [ 16/27] net: force a reload of first item in hlist_nulls_for_each_entry_rcu

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <eric.dumazet@...il.com>

[ Upstream commit c87a124a5d5e8cf8e21c4363c3372bcaf53ea190 ]

Roman Gushchin discovered that udp4_lib_lookup2() was not reloading
first item in the rcu protected list, in case the loop was restarted.

This produced soft lockups as in https://lkml.org/lkml/2013/4/16/37

rcu_dereference(X)/ACCESS_ONCE(X) seem to not work as intended if X is
ptr->field :

In some cases, gcc caches the value or ptr->field in a register.

Use a barrier() to disallow such caching, as documented in
Documentation/atomic_ops.txt line 114

Thanks a lot to Roman for providing analysis and numerous patches.

Diagnosed-by: Roman Gushchin <klamm@...dex-team.ru>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Boris Zhmurov <zhmurov@...dex-team.ru>
Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 include/linux/rculist_nulls.h |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -105,9 +105,14 @@ static inline void hlist_nulls_add_head_
  * @head:	the head for your list.
  * @member:	the name of the hlist_nulls_node within the struct.
  *
+ * The barrier() is needed to make sure compiler doesn't cache first element [1],
+ * as this loop can be restarted [2]
+ * [1] Documentation/atomic_ops.txt around line 114
+ * [2] Documentation/RCU/rculist_nulls.txt around line 146
  */
 #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)			\
-	for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));		\
+	for (({barrier();}),							\
+	     pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));		\
 		(!is_a_nulls(pos)) &&						\
 		({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
 		pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))


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