RE: [PATCH 7/8] vfio/mdev: Fix aborting mdev child device removal if one fails

[Parav Pandit <parav@mellanox.com>]

> -----Original Message-----
> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Monday, March 25, 2019 3:50 PM
> To: Kirti Wankhede <kwankhede@nvidia.com>
> Cc: Parav Pandit <parav@mellanox.com>; kvm@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH 7/8] vfio/mdev: Fix aborting mdev child device removal if
> one fails
> 
> On Tue, 26 Mar 2019 01:05:34 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
> > On 3/23/2019 4:50 AM, Parav Pandit 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
> > > device_for_each_child doesn't abort if one child removal hits error.
> > >
> >
> > When unregistering parent device, force_remove is set to true amd
> > mdev_device_remove_ops() always returns success.
> 
> Can we know that?  mdev_device_remove() doesn't guarantee to return
> zero.
> 
> > > 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().
> >
> > There is a need to pass the flag, pasting here the comment above
> > mdev_device_remove_ops() which explains why the flag is needed:
> >
> > /*
> >  * mdev_device_remove_ops gets called from sysfs's 'remove' and when
> > parent
> >  * device is being unregistered from mdev device framework.
> >  * - 'force_remove' is set to 'false' when called from sysfs's 'remove'
> > which
> >  *   indicates that if the mdev device is active, used by VMM or userspace
> >  *   application, vendor driver could return error then don't remove the
> > device.
> >  * - 'force_remove' is set to 'true' when called from
> > mdev_unregister_device()
> >  *   which indicate that parent device is being removed from mdev device
> >  *   framework so remove mdev device forcefully.
> >  */
> 
> I don't see that this changes the force behavior, it's simply noting that in
> order to continue the device_for_each_child() iterator, we need to return
> zero, regardless of what mdev_device_remove() returns, and the parent
> remove path is the only caller of mdev_device_remove_cb(), so we can
> assume force = true when calling mdev_device_remove().  Aside from maybe
> a WARN_ON if mdev_device_remove() returns non-zero, that much looks
> reasonable to me.
> 
> >  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().
> > >
> > > Hence, remove the unnecessary loop in mdev_device_remove().
> 
> I don't think knowing the device type is the only reason for this loop though.
> Both paths you mention above can race with each other, so we need to
> serialize them and pick a winner.  The mdev_list_lock allows us to do that.
> Additionally...
> 
> > >
> > > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > > ---
> > >  drivers/vfio/mdev/mdev_core.c | 24 +++++-------------------
> > >  1 file changed, 5 insertions(+), 19 deletions(-)
> > >
> > > diff --git a/drivers/vfio/mdev/mdev_core.c
> > > b/drivers/vfio/mdev/mdev_core.c index ab05464..944a058 100644
> > > --- a/drivers/vfio/mdev/mdev_core.c
> > > +++ b/drivers/vfio/mdev/mdev_core.c
> > > @@ -150,10 +150,10 @@ static int mdev_device_remove_ops(struct
> > > mdev_device *mdev, bool force_remove)
> > >
> > >  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;
> > >  }
> > >
> > >  /*
> > > @@ -241,7 +241,6 @@ 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); @@ -255,8 +254,7 @@ 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);
> > >
> > > @@ -346,24 +344,12 @@ int mdev_device_create(struct kobject *kobj,
> > > struct device *dev, uuid_le uuid)
> > >
> > >  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);
> 
> Acquiring the lock is removed, but...
> 
Crap. Missed the lower part.

> > > -	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;
> > >
> 
> We still release it in this path and the code below here.  If we don't find the
> device on the list under lock, then we're working with a stale device and
> playing with the 'active' flag of that device outside of any sort of mutual
> exclusion is racy.  Thanks,
Subsequent patch makes the order sane.
I think I should merge this change with patch-8 in the series.