RE: [PATCH net-next 11/19] vfio/mdev: Improvise mdev life cycle and parent removal scheme

Parav Pandit
2019-11-08T16:13:03Z

> -----Original Message-----
> From: Cornelia Huck <cohuck@redhat.com>
> Sent: Friday, November 8, 2019 7:01 AM
> To: Parav Pandit <parav@mellanox.com>
> Cc: alex.williamson@redhat.com; davem@davemloft.net;
> kvm@vger.kernel.org; netdev@vger.kernel.org; Saeed Mahameed
> <saeedm@mellanox.com>; kwankhede@nvidia.com; leon@kernel.org; Jiri
> Pirko <jiri@mellanox.com>; linux-rdma@vger.kernel.org
> Subject: Re: [PATCH net-next 11/19] vfio/mdev: Improvise mdev life cycle and
> parent removal scheme
>
> On Thu, 7 Nov 2019 10:08:26 -0600
> Parav Pandit <parav@mellanox.com> wrote:
>
> I guess that should be s/Improvise/improve/ in $SUBJECT, no?
>
Will do.

[..]

> >
> > To summarize,
> > mdev_remove()
> > read locks -> unreg_sem [ lock-A ]
> > [..]
> > devlink_unregister();
> > mutex lock devlink_mutex [ lock-B ]
> >
> > devlink eswitch->switchdev-legacy mode change.
> > devlink_nl_cmd_eswitch_set_doit()
> > mutex lock devlink_mutex [ lock-B ]
> > mdev_unregister_device()
> > write locks -> unreg_sem [ lock-A]
>
> So, this problem starts to pop up once you hook up that devlink stuff with
> the mdev stuff, and previous users of mdev just did not have a locking
> scheme similar to devlink?
>
Correct.

> >
> > Hence, instead of using semaphore, such synchronization is achieved
> > using srcu which is more flexible that eliminates nested locking.
> >
> > SRCU based solution is already proposed before at [2].
> >
> > [1] commit 5715c4dd66a3 ("vfio/mdev: Synchronize device create/remove
> > with parent removal") [2]
> > https://lore.kernel.org/patchwork/patch/1055254/
>
> I don't quite recall the discussion there... is this a rework of a patch you
> proposed before? Confused.
>
It was one huge patch, fixing multiple issues.
Alex suggested to split into multiple.
Initially for this issue I had it srcu, while redoing them to smaller patches, I guess for simplicity I moved to semaphore.
Once I enabled all my tested after a break, I realized that fix is not enough.

> >
> > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > ---
> > drivers/vfio/mdev/mdev_core.c | 56 +++++++++++++++++++++++---------
> > drivers/vfio/mdev/mdev_private.h | 3 +-
> > 2 files changed, 43 insertions(+), 16 deletions(-)
>
> (...)
>
> > @@ -207,6 +207,7 @@ int mdev_register_device(struct device *dev, const
> struct mdev_parent_ops *ops)
> > dev_warn(dev, "Failed to create compatibility class link\n");
> >
> > list_add(&parent->next, &parent_list);
> > + rcu_assign_pointer(parent->self, parent);
> > mutex_unlock(&parent_list_lock);
> >
> > dev_info(dev, "MDEV: Registered\n"); @@ -250,14 +251,29 @@ void
> > mdev_unregister_device(struct device *dev)
> > list_del(&parent->next);
> > mutex_unlock(&parent_list_lock);
> >
> > - down_write(&parent->unreg_sem);
> > + /*
> > + * Publish that this mdev parent is unregistering. So any new
> > + * create/remove cannot start on this parent anymore by user.
> > + */
> > + rcu_assign_pointer(parent->self, NULL);
> > +
> > + /*
> > + * Wait for any active create() or remove() mdev ops on the parent
> > + * to complete.
> > + */
> > + synchronize_srcu(&parent->unreg_srcu);
> > +
> > + /*
> > + * At this point it is confirmed that any pending user initiated
> > + * create or remove callbacks accessing the parent are completed.
> > + * It is safe to remove the parent now.
> > + */
>
> So, you're putting an srcu-handled self reference there and use that as an
> indication whether the parent is unregistering?
>
Right.