netdev - RE: [PATCH rdma-rc] RDMA/mlx5: Fix devlink deadlock on net namespace deletion

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Date:   Tue, 20 Oct 2020 11:41:50 +0000
From:   Parav Pandit <parav@...dia.com>
To:     Parav Pandit <parav@...dia.com>, Jason Gunthorpe <jgg@...dia.com>
CC:     Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jiri Pirko <jiri@...lanox.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Michael Guralnik <michaelgur@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...dia.com>
Subject: RE: [PATCH rdma-rc] RDMA/mlx5: Fix devlink deadlock on net namespace
 deletion

Hi Jason,

> From: Parav Pandit <parav@...dia.com>
> Sent: Tuesday, October 20, 2020 12:57 AM
> 
> 
> 
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Tuesday, October 20, 2020 12:31 AM
> >
> > On Mon, Oct 19, 2020 at 01:23:23PM +0000, Parav Pandit wrote:
> > > > > -	err = register_netdevice_notifier(&dev-
> >port[port_num].roce.nb);
> > > > > +	err = register_netdevice_notifier_net(mlx5_core_net(dev-
> >mdev),
> > > > > +					      &dev-
> >port[port_num].roce.nb);
> > > >
> > > > This looks racy, what lock needs to be held to keep
> > > > *mlx5_core_net()
> > stable?
> > >
> > > mlx5_core_net() cannot be accessed outside of mlx5 driver's load,
> > > unload,
> > reload path.
> > >
> > > When this is getting executed, devlink cannot be executing reload.
> > > This is guarded by devlink_reload_enable/disable calls done by mlx5 core.
> >
> > A comment that devlink_reload_enable/disable() must be held would be
> > helpful
> >
> Yes. will add.
> 
> > > >
> > > > >  	if (err) {
> > > > >  		dev->port[port_num].roce.nb.notifier_call = NULL;
> > > > >  		return err;
> > > > > @@ -3335,7 +3336,8 @@ static int mlx5_add_netdev_notifier(struct
> > > > >mlx5_ib_dev *dev, u8 port_num)  static void
> > > > >mlx5_remove_netdev_notifier(struct mlx5_ib_dev *dev, u8
> port_num)
> > {
> > > > >  	if (dev->port[port_num].roce.nb.notifier_call) {
> > > > > -		unregister_netdevice_notifier(&dev-
> > > > >port[port_num].roce.nb);
> > > > > +
> 	unregister_netdevice_notifier_net(mlx5_core_net(dev-
> > > > >mdev),
> > > > > +						  &dev-
> > > > >port[port_num].roce.nb);
> > > >
> > > > This seems dangerous too, what if the mlx5_core_net changed before
> > > > we get here?
> > >
> > > When I inspected driver, code, I am not aware of any code flow where
> > > this can change before reaching here, because registration and
> > > unregistration is done only in driver load, unload and reload path.
> > > Reload can happen only after devlink_reload_enable() is done.
> >
> > But we enable reload right after init_one
> >
> > > > What are the rules for when devlink_net() changes?
> > > >
> > > devlink_net() changes only after unload() callback is completed in driver.
> >
> > You mean mlx5_devlink_reload_down ?
> >
> Right.
> > That seems OK then
> Ok. will work with Leon to add the comment.

Is below fix up ok?

commit 33cf8a09e735849f622e8084a7b08d421f11a4e1 (HEAD -> netns-del-fix)
Author: Parav Pandit <parav@...dia.com>
Date:   Tue Oct 20 12:26:08 2020 +0300

    fixup: for RDMA/mlx5: Fix devlink deadlock on net namespace deletion

    Changelog:
    v0->v1:
     - Added kdoc comment description for the API usage and allowed context

    issue: 2230150
    Change-Id: Ibd233f771682c27565f48c54cd48fd87b0a7790f
    Signed-off-by: Parav Pandit <parav@...dia.com>

diff --git a/include/linux/mlx5/driver.h b/include/linux/mlx5/driver.h
index 560b551d5ff8..3382855b7ef1 100644
--- a/include/linux/mlx5/driver.h
+++ b/include/linux/mlx5/driver.h
@@ -1209,6 +1209,19 @@ static inline bool mlx5_is_roce_enabled(struct mlx5_core_dev *dev)
        return val.vbool;
 }

+/**
+ * mlx5_core_net - Provide net namespace of the mlx5_core_dev
+ * @dev: mlx5 core device
+ *
+ * mlx5_core_net() returns the net namespace of mlx5 core device.
+ * This can be called only in below described limited context.
+ * (a) When a devlink instance for mlx5_core is registered and
+ *     when devlink reload operation is disabled.
+ *     or
+ * (b) during devlink reload reload_down() and reload_up callbacks
+ *     where it is ensured that devlink instance's net namespace is
+ *     stable.
+ */
 static inline struct net *mlx5_core_net(struct mlx5_core_dev *dev)
 {
        return devlink_net(priv_to_devlink(dev));