Patchwork [v5,1/3] netpoll: add generic support for bridge and bonding devices

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Submitter Andy Gospodarek
Date June 4, 2010, 7:18 p.m.
Message ID <20100604191841.GM7497@gospo.rdu.redhat.com>
Download mbox | patch
Permalink /patch/54671/
State RFC
Delegated to: David Miller
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Comments

Andy Gospodarek - June 4, 2010, 7:18 p.m.
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@redhat.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.


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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Amerigo Wang - June 7, 2010, 9:57 a.m.
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@redhat.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.


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

Anyway, Flavio, could you try the following patch as well?

Thanks a lot!

>
> 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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David Miller - June 7, 2010, 10:01 a.m.
From: Cong Wang <amwang@redhat.com>
Date: Mon, 07 Jun 2010 17:57:49 +0800

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

Of course I don't like it. :-)

I suspect the locking scheme will need to be changed.

Besides, if we're going to hack this up and do write lock attempts in
the read locking paths, there is no point in using a rwlock any more.
And I'm personally in disfavor of all rwlock usage anyways (it dirties
the cacheline for readers just as equally for writers, and if the
critically protected code path is short enough, that shared cache
line atomic operation will be the predominant cost).

So I'd say, 1) make this a spinlock and 2) try to use RCU for the
read path.

That would fix everything.
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Andy Gospodarek - June 7, 2010, 1:03 p.m.
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@redhat.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.
>
>
> 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.

> Anyway, Flavio, could you try the following patch as well?
>
> Thanks a lot!
>
>>
>> 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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Amerigo Wang - June 8, 2010, 8:36 a.m.
On 06/07/10 18:01, David Miller wrote:
> From: Cong Wang<amwang@redhat.com>
> Date: Mon, 07 Jun 2010 17:57:49 +0800
>
>> Hmm, I still feel like this way is ugly, although it may work.
>> I guess David doesn't like it either.
>
> Of course I don't like it. :-)
>
> I suspect the locking scheme will need to be changed.
>
> Besides, if we're going to hack this up and do write lock attempts in
> the read locking paths, there is no point in using a rwlock any more.
> And I'm personally in disfavor of all rwlock usage anyways (it dirties
> the cacheline for readers just as equally for writers, and if the
> critically protected code path is short enough, that shared cache
> line atomic operation will be the predominant cost).
>
> So I'd say, 1) make this a spinlock and 2) try to use RCU for the
> read path.
>
> That would fix everything.

Yeah, agreed. Even not talking about netconsole, bonding code
does have locking problems, netconsole just makes this problem
clear.

I will try your suggestions above.

Thanks!
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Amerigo Wang - June 8, 2010, 8:38 a.m.
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@redhat.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.
>>
>>
>> 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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Patch

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: