Re: [PATCH 1/2] IB/ipoib: Fix mcast list locking

[Daniel Vacek]

On Mon, Dec 11, 2023 at 2:45 PM Leon Romanovsky <leon@xxxxxxxxxx> wrote:
>
> On Mon, Dec 11, 2023 at 02:04:24PM +0100, Daniel Vacek wrote:
> > We need an additional protection against list removal between ipoib_mcast_join_task()
> > and ipoib_mcast_dev_flush() in case the &priv->lock needs to be dropped while
> > iterating the &priv->multicast_list in ipoib_mcast_join_task(). If the mcast
> > is removed while the lock was dropped, the for loop spins forever resulting
> > in a hard lockup (as was reported on RHEL 4.18.0-372.75.1.el8_6 kernel):
> >
> >     Task A (kworker/u72:2 below)       | Task B (kworker/u72:0 below)
> >     -----------------------------------+-----------------------------------
> >     ipoib_mcast_join_task(work)        | ipoib_ib_dev_flush_light(work)
> >       spin_lock_irq(&priv->lock)       | __ipoib_ib_dev_flush(priv, ...)
> >       list_for_each_entry(mcast,       | ipoib_mcast_dev_flush(dev = priv->dev)
> >           &priv->multicast_list, list) |   mutex_lock(&priv->mcast_mutex)
> >         ipoib_mcast_join(dev, mcast)   |
> >           spin_unlock_irq(&priv->lock) |
> >                                        |   spin_lock_irqsave(&priv->lock, flags)
> >                                        |   list_for_each_entry_safe(mcast, tmcast,
> >                                        |                  &priv->multicast_list, list)
> >                                        |     list_del(&mcast->list);
> >                                        |     list_add_tail(&mcast->list, &remove_list)
> >                                        |   spin_unlock_irqrestore(&priv->lock, flags)
> >           spin_lock_irq(&priv->lock)   |
> >                                        |   ipoib_mcast_remove_list(&remove_list)
> >    (Here, mcast is no longer on the    |     list_for_each_entry_safe(mcast, tmcast,
> >     &priv->multicast_list and we keep  |                            remove_list, list)
> >     spinning on the &remove_list of the \ >>>  wait_for_completion(&mcast->done)
> >     other thread which is blocked and the|
> >     list is still valid on it's stack.)  | mutex_unlock(&priv->mcast_mutex)
> >
> > Fix this by adding mutex_lock(&priv->mcast_mutex) to ipoib_mcast_join_task().
>
> I don't entirely understand the issue and the proposed solution.
> There is only one spin_unlock_irq() in the middle of list_for_each_entry(mcast, &priv->multicast_list, list)
> and it is right before return statement which will break the loop. So
> how will loop spin forever?

There's another unlock/lock pair around ib_sa_join_multicast() call in
ipoib_mcast_join() no matter the outcome of the condition. The
ib_sa_join_multicast() cannot be called with the lock being held due
to GFP_KERNEL allocation can possibly sleep. That's what's causing the
issue.

Actually if you check the code, only if the mentioned condition is
false (and the loop is not broken and returned from
ipoib_mcast_join_task()) the lock is released and re-acquired,
creating the window for
ipoib_ib_dev_flush_light()/ipoib_mcast_dev_flush() to break the list.
The vmcore data shows/confirms that clearly.

--nX


> Thanks
>
> > Unfortunately we could not reproduce the lockup and confirm this fix but
> > based on the code review I think this fix should address such lockups.
> >
> > crash> bc 31
> > PID: 747      TASK: ff1c6a1a007e8000  CPU: 31   COMMAND: "kworker/u72:2"
> > --
> >     [exception RIP: ipoib_mcast_join_task+0x1b1]
> >     RIP: ffffffffc0944ac1  RSP: ff646f199a8c7e00  RFLAGS: 00000002
> >     RAX: 0000000000000000  RBX: ff1c6a1a04dc82f8  RCX: 0000000000000000
> >                                   work (&priv->mcast_task{,.work})
> >     RDX: ff1c6a192d60ac68  RSI: 0000000000000286  RDI: ff1c6a1a04dc8000
> >            &mcast->list
> >     RBP: ff646f199a8c7e90   R8: ff1c699980019420   R9: ff1c6a1920c9a000
> >     R10: ff646f199a8c7e00  R11: ff1c6a191a7d9800  R12: ff1c6a192d60ac00
> >                                                          mcast
> >     R13: ff1c6a1d82200000  R14: ff1c6a1a04dc8000  R15: ff1c6a1a04dc82d8
> >            dev                    priv (&priv->lock)     &priv->multicast_list (aka head)
> >     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
> > --- <NMI exception stack> ---
> >  #5 [ff646f199a8c7e00] ipoib_mcast_join_task+0x1b1 at ffffffffc0944ac1 [ib_ipoib]
> >  #6 [ff646f199a8c7e98] process_one_work+0x1a7 at ffffffff9bf10967
> >
> > crash> rx ff646f199a8c7e68
> > ff646f199a8c7e68:  ff1c6a1a04dc82f8 <<< work = &priv->mcast_task.work
> >
> > crash> list -hO ipoib_dev_priv.multicast_list ff1c6a1a04dc8000
> > (empty)
> >
> > crash> ipoib_dev_priv.mcast_task.work.func,mcast_mutex.owner.counter ff1c6a1a04dc8000
> >   mcast_task.work.func = 0xffffffffc0944910 <ipoib_mcast_join_task>,
> >   mcast_mutex.owner.counter = 0xff1c69998efec000
> >
> > crash> b 8
> > PID: 8        TASK: ff1c69998efec000  CPU: 33   COMMAND: "kworker/u72:0"
> > --
> >  #3 [ff646f1980153d50] wait_for_completion+0x96 at ffffffff9c7d7646
> >  #4 [ff646f1980153d90] ipoib_mcast_remove_list+0x56 at ffffffffc0944dc6 [ib_ipoib]
> >  #5 [ff646f1980153de8] ipoib_mcast_dev_flush+0x1a7 at ffffffffc09455a7 [ib_ipoib]
> >  #6 [ff646f1980153e58] __ipoib_ib_dev_flush+0x1a4 at ffffffffc09431a4 [ib_ipoib]
> >  #7 [ff646f1980153e98] process_one_work+0x1a7 at ffffffff9bf10967
> >
> > crash> rx ff646f1980153e68
> > ff646f1980153e68:  ff1c6a1a04dc83f0 <<< work = &priv->flush_light
> >
> > crash> ipoib_dev_priv.flush_light.func,broadcast ff1c6a1a04dc8000
> >   flush_light.func = 0xffffffffc0943820 <ipoib_ib_dev_flush_light>,
> >   broadcast = 0x0,
> >
> > The mcast(s) on the &remove_list (the remaining part of the ex &priv->multicast_list):
> >
> > crash> list -s ipoib_mcast.done.done ipoib_mcast.list -H ff646f1980153e10 | paste - -
> > ff1c6a192bd0c200        done.done = 0x0,
> > ff1c6a192d60ac00        done.done = 0x0,
> >
> > Reported-by: Yuya Fujita-bishamonten <fj-lsoft-rh-driver@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Daniel Vacek <neelx@xxxxxxxxxx>
> > ---
> >  drivers/infiniband/ulp/ipoib/ipoib_multicast.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/infiniband/ulp/ipoib/ipoib_multicast.c b/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> > index 5b3154503bf4..8e4f2c8839be 100644
> > --- a/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> > +++ b/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> > @@ -580,6 +580,7 @@ void ipoib_mcast_join_task(struct work_struct *work)
> >       }
> >       netif_addr_unlock_bh(dev);
> >
> > +     mutex_lock(&priv->mcast_mutex);
> >       spin_lock_irq(&priv->lock);
> >       if (!test_bit(IPOIB_FLAG_OPER_UP, &priv->flags))
> >               goto out;
> > @@ -634,6 +635,7 @@ void ipoib_mcast_join_task(struct work_struct *work)
> >                               /* Found the next unjoined group */
> >                               if (ipoib_mcast_join(dev, mcast)) {
> >                                       spin_unlock_irq(&priv->lock);
> > +                                     mutex_unlock(&priv->mcast_mutex);
> >                                       return;
> >                               }
> >                       } else if (!delay_until ||
> > @@ -655,6 +657,7 @@ void ipoib_mcast_join_task(struct work_struct *work)
> >               ipoib_mcast_join(dev, mcast);
> >
> >       spin_unlock_irq(&priv->lock);
> > +     mutex_unlock(&priv->mcast_mutex);
> >  }
> >
> >  void ipoib_mcast_start_thread(struct net_device *dev)
> > --
> > 2.43.0
> >
>