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

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Message-ID: <20111019114129.162d895a@nehalam.linuxnetplumber.net>
Date:	Wed, 19 Oct 2011 11:41:29 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Jay Vosburgh <fubar@...ibm.com>
Cc:	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,
	WANG Cong <xiyou.wangcong@...il.com>
Subject: Re: [PATCH net -v2] [BUGFIX] bonding: use flush_delayed_work_sync
 in bond_close

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...
              
                                    
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