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Date:	Sat, 2 Aug 2008 15:37:19 +0200
From:	Jarek Poplawski <>
To:	"Tantilov, Emil S" <>
Cc:	David Miller <>,
	"Kirsher, Jeffrey T" <>,
	"" <>
Subject: Re: [BUG] NULL pointer dereference in skb_dequeue

Tantilov, Emil S wrote, On 08/02/2008 11:36 AM:

> David Miller wrote:
>> Once we decide on a root qdisc to process, we shouldn't use
>> qdisc_root_lock() since that will resample qdisc->dev_queue->qdisc
>> which might be different.
>> This points out a core problem, and I might need to add a
>> root_qdisc backpointer to struct Qdisc to make this all work
>> out sanely for all cases.
>> Anyways, please try this patch:
> Still panics. Survived few MTU changes, but eventually I got this (see attached file - sorry for the partial dump, but that's all I can do remotely).

I guess this "root lock" has to go back to netdev_queue. Alas, I can't
test this, so if it's not a big problem maybe you could try this patch
before David goes back to this? (His patch should be removed before
using this one.)

Jarek P.


 include/linux/netdevice.h |    1 +
 include/net/sch_generic.h |    4 +---
 net/core/dev.c            |    1 +
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index ee583f6..5c32b70 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -447,6 +447,7 @@ struct netdev_queue {
 	struct net_device	*dev;
 	struct Qdisc		*qdisc;
 	unsigned long		state;
+	spinlock_t		queue_lock;
 	spinlock_t		_xmit_lock;
 	int			xmit_lock_owner;
 	struct Qdisc		*qdisc_sleeping;
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index b5f40d7..97ea112 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -195,9 +195,7 @@ static inline struct Qdisc *qdisc_root(struct Qdisc *qdisc)
 static inline spinlock_t *qdisc_root_lock(struct Qdisc *qdisc)
-	struct Qdisc *root = qdisc_root(qdisc);
-	return qdisc_lock(root);
+	return &qdisc->dev_queue->queue_lock;
 static inline struct net_device *qdisc_dev(struct Qdisc *qdisc)
diff --git a/net/core/dev.c b/net/core/dev.c
index 69320a5..73f3a65 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3861,6 +3861,7 @@ static void __netdev_init_queue_locks_one(struct net_device *dev,
 					  struct netdev_queue *dev_queue,
 					  void *_unused)
+	spin_lock_init(&dev_queue->queue_lock);
 	netdev_set_xmit_lockdep_class(&dev_queue->_xmit_lock, dev->type);
 	dev_queue->xmit_lock_owner = -1;
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