linux-kernel - Re: [PATCH net -v2] [BUGFIX] bonding: use flush_delayed_work_sync in bond

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Date:	Fri, 21 Oct 2011 13:45:32 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Jay Vosburgh <fubar@...ibm.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Mitsuo Hayasaka <mitsuo.hayasaka.hu@...achi.com>,
	Andy Gospodarek <andy@...yhouse.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH net -v2] [BUGFIX] bonding: use flush_delayed_work_sync in bond_close

On Thu, Oct 20, 2011 at 3:09 AM, Jay Vosburgh <fubar@...ibm.com> wrote:
> Stephen Hemminger <shemminger@...tta.com> wrote:
>
>>On Wed, 19 Oct 2011 11:01:02 -0700
>>Jay Vosburgh <fubar@...ibm.com> wrote:
>>
>>> Mitsuo Hayasaka <mitsuo.hayasaka.hu@...achi.com> 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:
>
> http://lists.openwall.net/netdev/2009/12/17/3
>
>        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?

Maybe I am too blind, why do we need rtnl_lock for cancel_delayed_work()
inside bond_close()?

Thanks.
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