Re: [v5 Patch 1/3] netpoll: add generic support for bridge and bondingdevices

From: Cong Wang
Date: Tue Jun 08 2010 - 04:35:22 EST

On 06/07/10 21:03, Andy Gospodarek wrote:
On Mon, Jun 07, 2010 at 05:57:49PM +0800, Cong Wang wrote:
On 06/05/10 03:18, Andy Gospodarek wrote:
On Wed, Jun 02, 2010 at 06:04:45PM +0800, Cong Wang wrote:
On 06/02/10 02:42, Jay Vosburgh wrote:
Cong Wang<amwang@xxxxxxxxxx> wrote:

On 06/01/10 03:08, Flavio Leitner wrote:
On Mon, May 31, 2010 at 01:56:52PM +0800, Cong Wang wrote:
Hi, Flavio,

Please use the attached patch instead, try to see if it solves
all your problems.

I tried and it hangs. No backtraces this time.
The bond_change_active_slave() prints before NETDEV_BONDING_FAILOVER
notification, so I think it won't work.

Ah, I thought the same.

Please, correct if I'm wrong, but when a failover happens with your
patch applied, the netconsole would be disabled forever even with
another healthy slave, right?

Yes, this is an easy solution, because bonding has several modes,
it is complex to make netpoll work in different modes.

If I understand correctly, the root cause of the problem with
netconsole and bonding is that bonding is, ultimately, performing
printks with a write lock held, and when netconsole recursively calls
into bonding to send the printk over the netconsole, there is a deadlock
(when the bonding xmit function attempts to acquire the same lock for


You're trying to avoid the deadlock by shutting off netconsole
(permanently, it looks like) for one problem case: a failover, which
does some printks with a write lock held.

This doesn't look to me like a complete solution, there are
other cases in bonding that will do printk with write locks held. I
suspect those will also hang netconsole as things exist today, and won't
be affected by your patch below.

I can expect that, bonding modes are complex.

For example:

The sysfs functions to set the primary (bonding_store_primary)
or active (bonding_store_active_slave) options: a pr_info is called to
provide a log message of the results. These could be tested by setting
the primary or active options via sysfs, e.g.,

echo eth0> /sys/class/net/bond0/bonding/primary
echo eth0> /sys/class/net/bond0/bonding/active

If the kernel is defined with DEBUG, there are a few pr_debug
calls within write_locks (bond_del_vlan, for example).

If the slave's underlying device driver's ndo_vlan_rx_register
or ndo_vlan_rx_kill_vid functions call printk (and it looks like some do
for error cases, e.g., igbvf, ehea, enic), those would also presumably
deadlock (because bonding holds its write_lock when calling the ndo_
vlan functions).

It also appears that (with the patch below) some nominally
normal usage patterns will immediately disable netconsole. The one that
comes to mind is if the primary= option is set (to "eth1" for this
example), but that slave not enslaved first (the slaves are added, say,
eth0 then eth1). In that situation, when the primary slave (eth1 here)
is added, the first thing that will happen is a failover, and that will
disable netconsole.

Thanks for your detailed explanation!

This is why I said bonding is complex. I guess we would have to adjust
netpoll code for different bonding cases, one solution seems not fix all.
I am not sure how much work to do, since I am not familiar with bonding
code. Maybe Andy can help?

Sorry I've been silent until now. This does seem quite similar to a
problem I've previously encountered when dealing with bonding+netpoll on
some old 2.6.9-based kernels. There is no guarantee the methods used
there will apply here, but I'll talk about them anyway.

As Flavio noticed, recursive calls into the bond transmit routines were
not a good idea. I discovered the same and worked around this issue by
checking to see if we could take the bond->lock for writing before
continuing. If we could not get, I wanted to signal that this should be
queued for transmission later. Based on the flow of netpoll_send_skb
(or possibly for another reason that is escaping me right now) I added
one of these checks in bond_poll_controller too. These aren't the
prettiest fixes, but seemed to work well for me when I did this work in
the past. I realize the differences are not that great compared to some
of the patches posted by Flavio, but I think they are worth trying.

Hmm, I still feel like this way is ugly, although it may work.
I guess David doesn't like it either.

Notice how I referred to it as a work-around? :)

It certainly isn't a great way to resolve the issue, but I wanted to
offer my opinon on the issue since you asked.

Sorry for my misunderstanding.
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