From: Veaceslav Falico <vfalico@redhat•com>
To: Ding Tianhong <dingtianhong@huawei•com>
Cc: Jay Vosburgh <fubar@us•ibm.com>,
Andy Gospodarek <andy@greyhouse•net>,
"David S. Miller" <davem@davemloft•net>,
Nikolay Aleksandrov <nikolay@redhat•com>,
Netdev <netdev@vger•kernel.org>
Subject: Re: [PATCH net-next v2 1/6] bonding: simplify and use RCU protection for 3ad xmit path
Date: Wed, 4 Sep 2013 12:18:24 +0200 [thread overview]
Message-ID: <20130904101823.GO1992@redhat.com> (raw)
In-Reply-To: <522700D1.5060805@huawei.com>
On Wed, Sep 04, 2013 at 05:43:45PM +0800, Ding Tianhong wrote:
...snip...
>+/**
>+ * IMPORTANT: bond_first/last_slave_rcu can return NULL in case of an empty list
>+ * Caller must hold rcu_read_lock
>+ */
>+#define bond_first_slave_rcu(bond) \
>+ list_first_or_null_rcu(&(bond)->slave_list, struct slave, list)
>+#define bond_last_slave_rcu(bond) \
>+ (list_empty(&(bond)->slave_list) ? NULL : \
>+ bond_to_slave_rcu((bond)->slave_list.prev))
Here, bond_last_slave_rcu() is racy. The list can be non-empty when
list_empty() is verified, however afterwards it might become empty, when
you call bond_to_slave_rcu(), and thus you'll get
bond_to_slave(bond->slave_list) in the result, which is not a slave.
Take a look at list_first_or_null_rcu() for a reference. The main idea is
that it first gets the ->next pointer, with RCU protection, and then
verifies if it's the list head or not, and if not - it gets the container
already. This way the ->next pointer won't get away.
These kind of bugs are really rare, but are *EXTREMELY* hard to debug.
>+
> #define bond_is_first_slave(bond, pos) ((pos)->list.prev == &(bond)->slave_list)
> #define bond_is_last_slave(bond, pos) ((pos)->list.next == &(bond)->slave_list)
>
>@@ -93,6 +106,15 @@
> (bond_is_first_slave(bond, pos) ? bond_last_slave(bond) : \
> bond_to_slave((pos)->list.prev))
>
>+/* Since bond_first/last_slave_rcu can return NULL, these can return NULL too */
>+#define bond_next_slave_rcu(bond, pos) \
>+ (bond_is_last_slave(bond, pos) ? bond_first_slave_rcu(bond) : \
>+ bond_to_slave_rcu((pos)->list.next))
>+
>+#define bond_prev_slave_rcu(bond, pos) \
>+ (bond_is_first_slave(bond, pos) ? bond_last_slave_rcu(bond) : \
>+ bond_to_slave_rcu((pos)->list.prev))
>+
These two are also racy. bond_is_last/first_slave() is not rcu-ified, and
thus you can't rely on it without proper locking. Same ideas apply as per
bond_first_slave_rcu().
next prev parent reply other threads:[~2013-09-04 10:19 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-04 9:43 [PATCH net-next v2 1/6] bonding: simplify and use RCU protection for 3ad xmit path Ding Tianhong
2013-09-04 10:18 ` Veaceslav Falico [this message]
2013-09-04 14:53 ` Ding Tianhong
2013-09-04 16:25 ` David Miller
2013-09-05 2:06 ` Ding Tianhong
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