Re: [PATCH net -v2] [BUGFIX] bonding: use flush_delayed_work_syncin bond_close

From: HAYASAKA Mitsuo
Date: Mon Oct 24 2011 - 00:01:00 EST

(2011/10/22 9:59), Jay Vosburgh wrote:
> Jay Vosburgh <fubar@xxxxxxxxxx> wrote:
>> AmÃrico Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>>> On Thu, Oct 20, 2011 at 3:09 AM, Jay Vosburgh <fubar@xxxxxxxxxx> wrote:
>>>> Stephen Hemminger <shemminger@xxxxxxxxxx> wrote:
>>>>> On Wed, 19 Oct 2011 11:01:02 -0700
>>>>> Jay Vosburgh <fubar@xxxxxxxxxx> wrote:
>>>>>> Mitsuo Hayasaka <> wrote:
>>>>>>> The bond_close() calls cancel_delayed_work() to cancel delayed works.
>>>>>>> It, however, cannot cancel works that were already queued in workqueue.
>>>>>>> The bond_open() initializes work->data, and proccess_one_work() refers
>>>>>>> get_work_cwq(work)->wq->flags. The get_work_cwq() returns NULL when
>>>>>>> work->data has been initialized. Thus, a panic occurs.
>>>>>>> This patch uses flush_delayed_work_sync() instead of cancel_delayed_work()
>>>>>>> in bond_close(). It cancels delayed timer and waits for work to finish
>>>>>>> execution. So, it can avoid the null pointer dereference due to the
>>>>>>> parallel executions of proccess_one_work() and initializing proccess
>>>>>>> of bond_open().
>>>>>> I'm setting up to test this. I have a dim recollection that we
>>>>>> tried this some years ago, and there was a different deadlock that
>>>>>> manifested through the flush path. Perhaps changes since then have
>>>>>> removed that problem.
>>>>>> -J
>>>>> Won't this deadlock on RTNL. The problem is that:
>>>>> CPU0 CPU1
>>>>> rtnl_lock
>>>>> bond_close
>>>>> delayed_work
>>>>> mii_work
>>>>> read_lock(bond->lock);
>>>>> read_unlock(bond->lock);
>>>>> rtnl_lock... waiting for CPU0
>>>>> flush_delayed_work_sync
>>>>> waiting for delayed_work to finish...
>>>> Yah, that was it. We discussed this a couple of years ago in
>>>> regards to a similar patch:
>>>> The short version is that we could rework the rtnl_lock inside
>>>> the montiors to be conditional and retry on failure (where "retry" means
>>>> "reschedule the work and try again later," not "spin retrying on rtnl").
>>>> That should permit the use of flush or cancel to terminate the work
>>>> items.
>>> Yes? Even if we use rtnl_trylock(), doesn't flush_delayed_work_sync()
>>> still queue the pending delayed work and wait for it to be finished?
>> Yes, it does. The original patch wants to use flush instead of
>> cancel to wait for the work to finish, because there's evidently a
>> possibility of getting back into bond_open before the work item
>> executes, and bond_open would reinitialize the work queue and corrupt
>> the queued work item.
>> The original patch series, and recipe for destruction, is here:
>> I've been unable to reproduce the work queue panic locally,
>> although it sounds plausible.
>> Mitsuo: can you provide the precise bonding configuration you're
>> using to induce the problem? Driver options, number and type of slaves,
>> etc.
>>> Maybe I am too blind, why do we need rtnl_lock for cancel_delayed_work()
>>> inside bond_close()?
>> We don't need RTNL for cancel/flush. However, bond_close is an
>> ndo_stop operation, and is called in the dev_close path, which always
>> occurs under RTNL. The mii / arp monitor work functions separately
>> acquire RTNL if they need to perform various failover related
>> operations.
>> I'm working on a patch that should resolve the mii / arp monitor
>> RTNL problem as I described above (if rtnl_trylock fails, punt and
>> reschedule the work). I need to rearrange the netdev_bonding_change
>> stuff a bit as well, since it acquires RTNL separately.
>> Once these changes are made to mii / arp monitor, then
>> bond_close can call flush instead of cancel, which should eliminate the
>> original problem described at the top.
> Just an update: there are three functions that may deadlock if
> the cancel work calls are changed to flush_sync. There are two
> rtnl_lock calls in each of the bond_mii_monitor and
> bond_activebackup_arp_mon functions, and one more in the
> bond_alb_monitor.
> Still testing to make sure I haven't missed anything, and I
> still haven't been able to reproduce Mitsuo's original failure.

The interval of mii_mon was set to 1 to reproduce this bug easily and
the 802.3ad mode was used. Then, I executed the following command.

# while true; do ifconfig bond0 down; done &
# while true; do ifconfig bond0 up; done &

This bug rarely occurs since it is the severe timing problem.
I found that it is more easily to reproduce this bug when using guest OS.

For example, it took one to three days for me to reproduce it on host OS,
but some hours on guest OS.


> -J
> ---
> -Jay Vosburgh, IBM Linux Technology Center, fubar@xxxxxxxxxx

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