Re: [RFC][PATCH] rcu in drivers/net/hamradio

From: Suzanne Wood
Date: Fri Oct 14 2005 - 11:51:00 EST

Thank you and please consider the following patch to drivers/net/hamradio/bpqether.c.

> From: "Herbert Xu"
> Sent: Friday, October 14, 2005 6:37 AM
> Suzanne Wood <suzannew@xxxxxxxxxx> wrote:
>> (1) bpq_new_device() calls list_add_rcu() labeled as
>> "list protected by RTNL." The comment may need to go
>> since the only apparent rtnl_lock()/unlock() pair encloses
>> the call to bpq_free_device() in bpq_cleanup_driver()
>> called upon module_exit().
> The RTNL comment is correct actually.
> bpq_new_device can only be called from bpq_device_event which
> is called from a netdev event handler. All netdev event handlers
> must be called uner the RTNL.

Thank you very much. I'll reinstate the comment and
submit this corrected patch
Signed-off-by: suzannew@xxxxxxxxxx

Because bpq_new_device() calls list_add_rcu()
and bpq_free_device() calls list_del_rcu(),
substitute list_for_each_entry_rcu() for
list_for_each_entry() in bpq_get_ax25_dev().
This requires the insertion of rcu_read_lock()/unlock().

A consequence of list_for_each_entry_rcu(bpq, &bpq_devices, bpq_list)
is that the future dereference of the pointer to the bpqdev
struct bpq is rcu-protected. But bpq_get_ax25_dev()
returns bpq->axdev, a pointer to a net_device struct. The
rcu_read_lock() in bpq_rcv() likely implies that is another
pointer to receive rcu-protected dereference.

The rcu_read_lock()/unlock() in bpq_device_event()
are removed due to the following found by considering
the cases of the switch statement:

(1) bpq_new_device() calls list_add_rcu() labeled as
"list protected by RTNL."

(2) dev_close() as defined in net/core/dev.c
employs a memory barrier.

(3) bpq_free_device() calls list_del_rcu() which, according
to list.h, requires synchronize_rcu() which can block or
call_rcu() or call_rcu_bh() which cannot block.
None of these is called anywhere in the directory drivers/net,
so synchronize_irq() might address this.


bpqether.c | 11 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)


--- src/linux-2.6.14-rc4/drivers/net/hamradio/bpqether.c 2005-10-10 18:19:19.000000000 -0700
+++ patch/linux-2.6.14-rc4/drivers/net/hamradio/bpqether.c 2005-10-14 08:56:12.000000000 -0700
@@ -144,10 +144,13 @@ static inline struct net_device *bpq_get
struct bpqdev *bpq;

- list_for_each_entry(bpq, &bpq_devices, bpq_list) {
+ rcu_read_lock();
+ list_for_each_entry_rcu(bpq, &bpq_devices, bpq_list) {
if (bpq->ethdev == dev)
+ rcu_read_unlock();
return bpq->axdev;
+ rcu_read_unlock();
return NULL;

@@ -179,7 +182,7 @@ static int bpq_rcv(struct sk_buff *skb,
goto drop;

- dev = bpq_get_ax25_dev(dev);
+ dev = rcu_dereference(bpq_get_ax25_dev(dev));

if (dev == NULL || !netif_running(dev))
goto drop_unlock;
@@ -561,8 +564,6 @@ static int bpq_device_event(struct notif
if (!dev_is_ethdev(dev))

- rcu_read_lock();
switch (event) {
case NETDEV_UP: /* new ethernet device -> new BPQ interface */
if (bpq_get_ax25_dev(dev) == NULL)
@@ -581,7 +582,6 @@ static int bpq_device_event(struct notif
- rcu_read_unlock();

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