Message-ID: <20100604191841.GM7497@gospo.rdu.redhat.com>
Date:	Fri, 4 Jun 2010 15:18:41 -0400
From:	Andy Gospodarek <andy@...yhouse.net>
To:	Cong Wang <amwang@...hat.com>
Cc:	Jay Vosburgh <fubar@...ibm.com>, Flavio Leitner <fbl@...close.org>,
	linux-kernel@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
	netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
	Andy Gospodarek <gospo@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	bonding-devel@...ts.sourceforge.net,
	David Miller <davem@...emloft.net>
Subject: Re: [v5 Patch 1/3] netpoll: add generic support for bridge and
	bonding devices

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@...hat.com>  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
>> read).
>
>
> Yes.
>
>>
>> 	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.

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index ef60244..d7b9b99 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -1290,6 +1290,12 @@ static bool slaves_support_netpoll(struct net_device *bond_dev)
 static void bond_poll_controller(struct net_device *bond_dev)
 {
 	struct net_device *dev = bond_dev->npinfo->netpoll->real_dev;
+	struct bonding *bond = netdev_priv(bond_dev);
+
+	if (!write_trylock(&bond->lock))
+		return;
+	write_unlock(&bond->lock);
+
 	if (dev != bond_dev)
 		netpoll_poll_dev(dev);
 }
@@ -4418,7 +4424,11 @@ static void bond_set_xmit_hash_policy(struct bonding *bond)
 
 static netdev_tx_t bond_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
-	const struct bonding *bond = netdev_priv(dev);
+	struct bonding *bond = netdev_priv(dev);
+
+	if (!write_trylock(&bond->lock))
+		return NETDEV_TX_BUSY;
+	write_unlock(&bond->lock);
 
 	switch (bond->params.mode) {
 	case BOND_MODE_ROUNDROBIN:

The other key to all of this is to make sure that queuing is done
correctly now that we expect to queue these frames and have them sent at
some point when there is a member of the bond that is actually capable
of sending them out.

The new style of sending queued skbs in a workqueue is much better than
what was done in the 2.6.9 timeframe, but careful attention should still
be paid to txq lock and which processor is the owner.  Returning
something other than NETDEV_TX_OK from bond_start_xmit and checking for
locks being held there should also help with any deadlocks that show up
while running in queue_process (though they would not be recursive).

I'm not in a good spot to test this right now, but I can take a look at
next week and we can try and track down any of the other deadlocks that
currently exist as I suspect this will not resolve all of the issues.
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