Re: [PATCH 4/8] IPoIB: fix mcast_dev_flush/mcast_restart_task race

[Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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On Fri, 2014-08-29 at 14:53 -0700, Wendy Cheng wrote:
> On Tue, Aug 12, 2014 at 4:38 PM, Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> > Our mcast_dev_flush routine and our mcast_restart_task can race against
> > each other.  In particular, they both hold the priv->lock while
> > manipulating the rbtree and while removing mcast entries from the
> > multicast_list and while adding entries to the remove_list, but they
> > also both drop their locks prior to doing the actual removes.  The
> > mcast_dev_flush routine is run entirely under the rtnl lock and so has
> > at least some locking.  The actual race condition is like this:
> >
> > Thread 1                                Thread 2
> > ifconfig ib0 up
> >   start multicast join for broadcast
> >   multicast join completes for broadcast
> >   start to add more multicast joins
> >     call mcast_restart_task to add new entries
> >                                         ifconfig ib0 down
> >                                           mcast_dev_flush
> >                                             mcast_leave(mcast A)
> >     mcast_leave(mcast A)
> >
> > As mcast_leave calls ib_sa_multicast_leave, and as member in
> > core/multicast.c is ref counted, we run into an unbalanced refcount
> > issue.  To avoid stomping on each others removes, take the rtnl lock
> > specifically when we are deleting the entries from the remove list.
> 
> Isn't "test_and_clear_bit()" atomic so it is unlikely that
> ib_sa_free_multicast() can run multiple times  ?
> 
>     638 static int ipoib_mcast_leave(struct net_device *dev, struct
> ipoib_mcast         *mcast)
>     639 {
>     640         struct ipoib_dev_priv *priv = netdev_priv(dev);
>     641         int ret = 0;
>     642
>     643         if (test_and_clear_bit(IPOIB_MCAST_FLAG_BUSY, &mcast->flags))
>     644                 ib_sa_free_multicast(mcast->mc);
>     645
>     646         if (test_and_clear_bit(IPOIB_MCAST_FLAG_ATTACHED,
> &mcast->flags)        ) {

This is the original code, and you would be right, but it had other
problems.  Even though test_and_clear_bit is atomic, the original usage
of MCAST_FLAG_BUSY was really that the mcast join had started, there was
no guarantee it had completed (we would set the flag, call
ib_sa_multicast_join, and we could then race with our leave where it was
possible to leave prior to the join completing).

My patches change this around now, and the code only checks for
MCAST_FLAG_BUSY as a double check that we aren't leaving an in-process
join.

> 
> -- Wendy
> 
> >
> > Signed-off-by: Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> > ---
> >  drivers/infiniband/ulp/ipoib/ipoib_multicast.c | 37 ++++++++++++++++++++++----
> >  1 file changed, 32 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/infiniband/ulp/ipoib/ipoib_multicast.c b/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> > index f5e8da530d9..19e3fe75ebf 100644
> > --- a/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> > +++ b/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> > @@ -810,7 +810,10 @@ void ipoib_mcast_dev_flush(struct net_device *dev)
> >
> >         spin_unlock_irqrestore(&priv->lock, flags);
> >
> > -       /* seperate between the wait to the leave*/
> > +       /*
> > +        * make sure the in-flight joins have finished before we attempt
> > +        * to leave
> > +        */
> >         list_for_each_entry_safe(mcast, tmcast, &remove_list, list)
> >                 if (test_bit(IPOIB_MCAST_FLAG_BUSY, &mcast->flags))
> >                         wait_for_completion(&mcast->done);
> > @@ -931,14 +934,38 @@ void ipoib_mcast_restart_task(struct work_struct *work)
> >         netif_addr_unlock(dev);
> >         local_irq_restore(flags);
> >
> > -       /* We have to cancel outside of the spinlock */
> > +       /*
> > +        * make sure the in-flight joins have finished before we attempt
> > +        * to leave
> > +        */
> > +       list_for_each_entry_safe(mcast, tmcast, &remove_list, list)
> > +               if (test_bit(IPOIB_MCAST_FLAG_BUSY, &mcast->flags))
> > +                       wait_for_completion(&mcast->done);
> > +
> > +       /*
> > +        * We have to cancel outside of the spinlock, but we have to
> > +        * take the rtnl lock or else we race with the removal of
> > +        * entries from the remove list in mcast_dev_flush as part
> > +        * of ipoib_stop() which will call mcast_stop_thread with
> > +        * flush == 1 while holding the rtnl lock, and the
> > +        * flush_workqueue won't complete until this restart_mcast_task
> > +        * completes.  So do like the carrier on task and attempt to
> > +        * take the rtnl lock, but if we can't before the ADMIN_UP flag
> > +        * goes away, then just return and know that the remove list will
> > +        * get flushed later by mcast_dev_flush.
> > +        */
> > +       while (!rtnl_trylock()) {
> > +               if (!test_bit(IPOIB_FLAG_ADMIN_UP, &priv->flags))
> > +                       return;
> > +               else
> > +                       msleep(20);
> > +       }
> >         list_for_each_entry_safe(mcast, tmcast, &remove_list, list) {
> >                 ipoib_mcast_leave(mcast->dev, mcast);
> >                 ipoib_mcast_free(mcast);
> >         }
> > -
> > -       if (test_bit(IPOIB_FLAG_ADMIN_UP, &priv->flags))
> > -               ipoib_mcast_start_thread(dev);
> > +       ipoib_mcast_start_thread(dev);
> > +       rtnl_unlock();
> >  }
> >
> >  #ifdef CONFIG_INFINIBAND_IPOIB_DEBUG
> > --
> > 1.9.3
> >
> > --
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-- 
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
              GPG KeyID: 0E572FDD



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