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

[Parav Pandit <parav@mellanox.com>]

> -----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.