RE: [PATCHv1 6/7] vfio/mdev: Fix aborting mdev child device removal if one fails

[Parav Pandit]

> -----Original Message-----
> From: Cornelia Huck <cohuck@xxxxxxxxxx>
> Sent: Monday, April 1, 2019 12:39 PM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> kwankhede@xxxxxxxxxx; alex.williamson@xxxxxxxxxx; cjia@xxxxxxxxxx
> Subject: Re: [PATCHv1 6/7] vfio/mdev: Fix aborting mdev child device
> removal if one fails
> 
> On Tue, 26 Mar 2019 22:45:44 -0500
> Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> 
> > device_for_each_child() stops executing callback function for
> > remaining child devices, if callback hits an error.
> > Each child mdev device is independent of each other.
> > While unregistering parent device, mdev core must remove all child
> > mdev devices.
> > Therefore, mdev_device_remove_cb() always returns success so that
> 
> s/always returns/must always return/ ?
> 
Must always return.
:-)

> > device_for_each_child doesn't abort if one child removal hits error.
> >
> > While at it, improve remove and unregister functions for below simplicity.
> >
> > There isn't need to pass forced flag pointer during mdev parent
> > removal which invokes mdev_device_remove(). So simplify the flow.
> >
> > mdev_device_remove() is called from two paths.
> > 1. mdev_unregister_driver()
> >      mdev_device_remove_cb()
> >        mdev_device_remove()
> > 2. remove_store()
> >      mdev_device_remove()
> >
> > When device is removed by user using remote_store(), device under
> > removal is mdev device.
> > When device is removed during parent device removal using generic
> > child iterator, mdev check is already done using dev_is_mdev().
> 
> Isn't there still a possible race condition (which you seem to address with
> the following patch)? IOW, you cannot remove that loop-under-mutex yet?

The loop checks if the remove() is called on the mdev or not.
This is already checked from both the paths from remove is invoked.
I didn't remove the 'active' check. So it should be fine.

> >
> > Hence, remove the unnecessary loop in mdev_device_remove().
> >
> > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > Signed-off-by: Parav Pandit <parav@xxxxxxxxxxxx>
> > ---
> >  drivers/vfio/mdev/mdev_core.c | 23 +++++------------------
> >  1 file changed, 5 insertions(+), 18 deletions(-)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c
> > b/drivers/vfio/mdev/mdev_core.c index 836d319..aefcf34 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -149,10 +149,10 @@ static int mdev_device_remove_ops(struct
> > mdev_device *mdev, bool force_remove)
> >
> 
> Maybe add
> 
> /* only called during parent device unregistration */
> 
> to avoid headscratching in the future?
> 
> >  static int mdev_device_remove_cb(struct device *dev, void *data)  {
> > -	if (!dev_is_mdev(dev))
> > -		return 0;
> > +	if (dev_is_mdev(dev))
> > +		mdev_device_remove(dev, true);
> >
> > -	return mdev_device_remove(dev, data ? *(bool *)data : true);
> > +	return 0;
> >  }
> >
> >  /*
> > @@ -240,7 +240,6 @@ int mdev_register_device(struct device *dev, const
> > struct mdev_parent_ops *ops)  void mdev_unregister_device(struct
> > device *dev)  {
> >  	struct mdev_parent *parent;
> > -	bool force_remove = true;
> >
> >  	mutex_lock(&parent_list_lock);
> >  	parent = __find_parent_device(dev);
> > @@ -254,8 +253,7 @@ void mdev_unregister_device(struct device *dev)
> >  	list_del(&parent->next);
> >  	class_compat_remove_link(mdev_bus_compat_class, dev, NULL);
> >
> > -	device_for_each_child(dev, (void *)&force_remove,
> > -			      mdev_device_remove_cb);
> > +	device_for_each_child(dev, NULL, mdev_device_remove_cb);
> >
> >  	parent_remove_sysfs_files(parent);
> >
> 
> Up to this chunk, the patch looks good to me.
> 
> > @@ -348,24 +346,13 @@ int mdev_device_create(struct kobject *kobj,
> >
> >  int mdev_device_remove(struct device *dev, bool force_remove)  {
> > -	struct mdev_device *mdev, *tmp;
> > +	struct mdev_device *mdev;
> >  	struct mdev_parent *parent;
> >  	struct mdev_type *type;
> >  	int ret;
> >
> >  	mdev = to_mdev_device(dev);
> > -
> >  	mutex_lock(&mdev_list_lock);
> > -	list_for_each_entry(tmp, &mdev_list, next) {
> > -		if (tmp == mdev)
> > -			break;
> > -	}
> > -
> > -	if (tmp != mdev) {
> > -		mutex_unlock(&mdev_list_lock);
> > -		return -ENODEV;
> > -	}
> > -
> >  	if (!mdev->active) {
> >  		mutex_unlock(&mdev_list_lock);
> >  		return -EAGAIN;