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    SubjectRe: [PATCH net -v2] [BUGFIX] bonding: use flush_delayed_work_sync in bond_close
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    On Thu, Oct 20, 2011 at 3:09 AM, Jay Vosburgh <fubar@us.ibm.com> wrote:
    > Stephen Hemminger <shemminger@vyatta.com> wrote:
    >
    >>On Wed, 19 Oct 2011 11:01:02 -0700
    >>Jay Vosburgh <fubar@us.ibm.com> wrote:
    >>
    >>> Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.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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