netdev - [PATCH]: qdisc_root_lock() requires RTNML

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Message-Id: <20080802.235537.203812172.davem@davemloft.net>
Date:	Sat, 02 Aug 2008 23:55:37 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	netdev@...r.kernel.org
Subject: [PATCH]: qdisc_root_lock() requires RTNML


Another followup after:

http://marc.info/?l=linux-netdev&m=121763401922311&w=2

Committed to net-2.6

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

pkt_sched: Make sure RTNL is held in qdisc_root_lock().

It is the only legal environment in which this can be
used.

Add some commentary explaining the situation.

Signed-off-by: David S. Miller <davem@...emloft.net>
---
 include/net/sch_generic.h |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index b5f40d7..c5bb130 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -193,10 +193,22 @@ static inline struct Qdisc *qdisc_root(struct Qdisc *qdisc)
 	return qdisc->dev_queue->qdisc;
 }
 
+/* The qdisc root lock is a mechanism by which to top level
+ * of a qdisc tree can be locked from any qdisc node in the
+ * forest.  This allows changing the configuration of some
+ * aspect of the qdisc tree while blocking out asynchronous
+ * qdisc access in the packet processing paths.
+ *
+ * It is only legal to do this when the root will not change
+ * on us.  Otherwise we'll potentially lock the wrong qdisc
+ * root.  This is enforced by holding the RTNL semaphore, which
+ * all users of this lock accessor must do.
+ */
 static inline spinlock_t *qdisc_root_lock(struct Qdisc *qdisc)
 {
 	struct Qdisc *root = qdisc_root(qdisc);
 
+	ASSERT_RTNL();
 	return qdisc_lock(root);
 }
 
-- 
1.5.6.GIT


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