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

[AmÃrico Wang]

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 <mitsuo.hayasaka.hu@xxxxxxxxxxx> 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().
>>>
>>> Â Â ÂI'm setting up to test this. ÂI have a dim recollection that we
>>> tried this some years ago, and there was a different deadlock that
>>> manifested through the flush path. ÂPerhaps changes since then have
>>> removed that problem.
>>>
>>> Â Â Â-J
>>
>>Won't this deadlock on RTNL. ÂThe problem is that:
>>
>> Â CPU0 Â Â Â Â Â Â Â Â Â Â Â Â Â ÂCPU1
>> Ârtnl_lock
>> Â Â Âbond_close
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â delayed_work
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â mii_work
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â read_lock(bond->lock);
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â read_unlock(bond->lock);
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â rtnl_lock... waiting for CPU0
>> Â Â Âflush_delayed_work_sync
>> Â Â Â Â Âwaiting for delayed_work to finish...
>
> Â Â Â ÂYah, that was it. ÂWe discussed this a couple of years ago in
> regards to a similar patch:
>
> http://lists.openwall.net/netdev/2009/12/17/3
>
> Â Â Â ÂThe 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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