RE: [PATCH 8/8] vfio/mdev: Improve the create/remove sequence

[Parav Pandit]

> -----Original Message-----
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Monday, March 25, 2019 6:19 PM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> kwankhede@xxxxxxxxxx
> Subject: Re: [PATCH 8/8] vfio/mdev: Improve the create/remove sequence
> 
> On Fri, 22 Mar 2019 18:20:35 -0500
> Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> 
> > There are five problems with current code structure.
> > 1. mdev device is placed on the mdev bus before it is created in the
> > vendor driver. Once a device is placed on the mdev bus without
> > creating its supporting underlying vendor device, an open() can get
> > triggered by userspace on partially initialized device.
> > Below ladder diagram highlight it.
> >
> >       cpu-0                                       cpu-1
> >       -----                                       -----
> >    create_store()
> >      mdev_create_device()
> >        device_register()
> >           ...
> >          vfio_mdev_probe()
> >          ...creates char device
> >                                         vfio_mdev_open()
> >                                           parent->ops->open(mdev)
> >                                             vfio_ap_mdev_open()
> >                                               matrix_mdev = NULL
> >         [...]
> >         parent->ops->create()
> >           vfio_ap_mdev_create()
> >             mdev_set_drvdata(mdev, matrix_mdev);
> >             /* Valid pointer set above */
> >
> > 2. Current creation sequence is,
> >    parent->ops_create()
> >    groups_register()
> >
> > Remove sequence is,
> >    parent->ops->remove()
> >    groups_unregister()
> > However, remove sequence should be exact mirror of creation sequence.
> > Once this is achieved, all users of the mdev will be terminated first
> > before removing underlying vendor device.
> > (Follow standard linux driver model).
> > At that point vendor's remove() ops shouldn't failed because device is
> > taken off the bus that should terminate the users.
> >
> > 3. Additionally any new mdev driver that wants to work on mdev device
> > during probe() routine registered using mdev_register_driver() needs
> > to get stable mdev structure.
> >
> > 4. In following sequence, child devices created while removing mdev
> > parent device can be left out, or it may lead to race of removing half
> > initialized child mdev devices.
> >
> > issue-1:
> > --------
> >        cpu-0                         cpu-1
> >        -----                         -----
> >                                   mdev_unregister_device()
> >                                      device_for_each_child()
> >                                         mdev_device_remove_cb()
> >                                             mdev_device_remove()
> > create_store()
> >   mdev_device_create()                   [...]
> >        device_register()
> >                                   parent_remove_sysfs_files()
> >                                   /* BUG: device added by cpu-0
> >                                    * whose parent is getting removed.
> >                                    */
> >
> > issue-2:
> > --------
> >        cpu-0                         cpu-1
> >        -----                         -----
> > create_store()
> >   mdev_device_create()                   [...]
> >        device_register()
> >
> >        [...]                      mdev_unregister_device()
> >                                      device_for_each_child()
> >                                         mdev_device_remove_cb()
> >                                             mdev_device_remove()
> >
> >        mdev_create_sysfs_files()
> >        /* BUG: create is adding
> >         * sysfs files for a device
> >         * which is undergoing removal.
> >         */
> >                                  parent_remove_sysfs_files()
> 
> In both cases above, it looks like the device will hold a reference to the
> parent, so while there is a race, the parent object isn't released.
Yes, parent object is not released but parent fields are not stable.

> 
> >
> > 5. Below crash is observed when user initiated remove is in progress
> > and mdev_unregister_driver() completes parent unregistration.
> >
> >        cpu-0                         cpu-1
> >        -----                         -----
> > remove_store()
> >    mdev_device_remove()
> >    active = false;
> >                                   mdev_unregister_device()
> >                                     remove type
> >    [...]
> >    mdev_remove_ops() crashes.
> >
> > This is similar race like create() racing with mdev_unregister_device().
> 
> Not sure I catch this, the device should have a reference to the parent, and
> we don't specifically clear parent->ops, so what's getting removed that
> causes this oops?  Is .remove pointing at bad text regardless?
> 
I guess the mdev_attr_groups being stale now.

> > mtty mtty: MDEV: Registered
> > iommu: Adding device 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001 to group 57
> > vfio_mdev 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001: MDEV: group_id = 57
> > mdev_device_remove sleep started mtty mtty: MDEV: Unregistering
> > mtty_dev: Unloaded!
> > BUG: unable to handle kernel paging request at ffffffffc027d668 PGD
> > af9818067 P4D af9818067 PUD af981a067 PMD 8583c3067 PTE 0
> > Oops: 0000 [#1] SMP PTI
> > CPU: 15 PID: 3517 Comm: bash Kdump: loaded Not tainted
> > 5.0.0-rc7-vdevbus+ #2 Hardware name: Supermicro
> > SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
> > RIP: 0010:mdev_device_remove_ops+0x1a/0x50 [mdev] Call Trace:
> >  mdev_device_remove+0xef/0x130 [mdev]
> >  remove_store+0x77/0xa0 [mdev]
> >  kernfs_fop_write+0x113/0x1a0
> >  __vfs_write+0x33/0x1b0
> >  ? rcu_read_lock_sched_held+0x64/0x70
> >  ? rcu_sync_lockdep_assert+0x2a/0x50
> >  ? __sb_start_write+0x121/0x1b0
> >  ? vfs_write+0x17c/0x1b0
> >  vfs_write+0xad/0x1b0
> >  ? trace_hardirqs_on_thunk+0x1a/0x1c
> >  ksys_write+0x55/0xc0
> >  do_syscall_64+0x5a/0x210
> >
> > Therefore, mdev core is improved in following ways to overcome above
> > issues.
> >
> > 1. Before placing mdev devices on the bus, perform vendor drivers
> > creation which supports the mdev creation.
> > This ensures that mdev specific all necessary fields are initialized
> > before a given mdev can be accessed by bus driver.
> >
> > 2. During remove flow, first remove the device from the bus. This
> > ensures that any bus specific devices and data is cleared.
> > Once device is taken of the mdev bus, perform remove() of mdev from
> > the vendor driver.
> >
> > 3. Linux core device model provides way to register and auto
> > unregister the device sysfs attribute groups at dev->groups.
> > Make use of this groups to let core create the groups and simplify
> > code to avoid explicit groups creation and removal.
> >
> > 4. Wait for any ongoing mdev create() and remove() to finish before
> > unregistering parent device using srcu. This continues to allow
> > multiple create and remove to progress in parallel. At the same time
> > guard parent removal while parent is being access by create() and remove
> callbacks.
> 
> So there should be 4-5 separate patches here?  Wishful thinking?
> 
create, remove racing with unregister is handled using srcu.
Change-3 cannot be done without fixing the sequence so it should be in patch that fixes it.
Change described changes 1-2-3 are just one change. It is just the patch description to bring clarity.
Change-4 can be possibly done as split to different patch.

> > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > Signed-off-by: Parav Pandit <parav@xxxxxxxxxxxx>
> > ---
> >  drivers/vfio/mdev/mdev_core.c    | 142 +++++++++++++++++++++--------------
> ----
> >  drivers/vfio/mdev/mdev_private.h |   7 +-
> >  drivers/vfio/mdev/mdev_sysfs.c   |   6 +-
> >  3 files changed, 84 insertions(+), 71 deletions(-)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c
> > b/drivers/vfio/mdev/mdev_core.c index 944a058..8fe0ed1 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -84,6 +84,7 @@ static void mdev_release_parent(struct kref *kref)
> >  						  ref);
> >  	struct device *dev = parent->dev;
> >
> > +	cleanup_srcu_struct(&parent->unreg_srcu);
> >  	kfree(parent);
> >  	put_device(dev);
> >  }
> > @@ -103,56 +104,30 @@ static inline void mdev_put_parent(struct
> mdev_parent *parent)
> >  		kref_put(&parent->ref, mdev_release_parent);  }
> >
> > -static int mdev_device_create_ops(struct kobject *kobj,
> > -				  struct mdev_device *mdev)
> > +static int mdev_device_must_remove(struct mdev_device *mdev)
> 
> Naming is off here, mdev_device_remove_common()?
> 
Yes, sounds better.

> >  {
> > -	struct mdev_parent *parent = mdev->parent;
> > +	struct mdev_parent *parent;
> > +	struct mdev_type *type;
> >  	int ret;
> >
> > -	ret = parent->ops->create(kobj, mdev);
> > -	if (ret)
> > -		return ret;
> > +	type = to_mdev_type(mdev->type_kobj);
> >
> > -	ret = sysfs_create_groups(&mdev->dev.kobj,
> > -				  parent->ops->mdev_attr_groups);
> > +	mdev_remove_sysfs_files(&mdev->dev, type);
> > +	device_del(&mdev->dev);
> > +	parent = mdev->parent;
> > +	ret = parent->ops->remove(mdev);
> >  	if (ret)
> > -		parent->ops->remove(mdev);
> > +		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
> 
> Let the caller decide whether to be verbose with the error, parent removal
> might want to warn, sysfs remove might just return an error.
> 
I didn't follow. Caller meaning mdev_device_remove_common() or vendor driver?

> >
> > +	/* Balances with device_initialize() */
> > +	put_device(&mdev->dev);
> >  	return ret;
> >  }
> >
> > -/*
> > - * 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.
> > - */
> > -static int mdev_device_remove_ops(struct mdev_device *mdev, bool
> > force_remove) -{
> > -	struct mdev_parent *parent = mdev->parent;
> > -	int ret;
> > -
> > -	/*
> > -	 * Vendor driver can return error if VMM or userspace application is
> > -	 * using this mdev device.
> > -	 */
> > -	ret = parent->ops->remove(mdev);
> > -	if (ret && !force_remove)
> > -		return ret;
> > -
> > -	sysfs_remove_groups(&mdev->dev.kobj, parent->ops-
> >mdev_attr_groups);
> > -	return 0;
> > -}
> 
> Seems like there's easily a separate patch in pushing the create/remove ops
> into the calling function and separating for the iterator callback, that would
> make this easier to review.
> 
> > -
> >  static int mdev_device_remove_cb(struct device *dev, void *data)  {
> >  	if (dev_is_mdev(dev))
> > -		mdev_device_remove(dev, true);
> > -
> > +		mdev_device_must_remove(to_mdev_device(dev));
> >  	return 0;
> >  }
> >
> > @@ -194,6 +169,7 @@ int mdev_register_device(struct device *dev, const
> struct mdev_parent_ops *ops)
> >  	}
> >
> >  	kref_init(&parent->ref);
> > +	init_srcu_struct(&parent->unreg_srcu);
> >
> >  	parent->dev = dev;
> >  	parent->ops = ops;
> > @@ -214,6 +190,7 @@ int mdev_register_device(struct device *dev, const
> struct mdev_parent_ops *ops)
> >  	if (ret)
> >  		dev_warn(dev, "Failed to create compatibility class link\n");
> >
> > +	rcu_assign_pointer(parent->self, parent);
> >  	list_add(&parent->next, &parent_list);
> >  	mutex_unlock(&parent_list_lock);
> >
> > @@ -244,21 +221,36 @@ void mdev_unregister_device(struct device *dev)
> >
> >  	mutex_lock(&parent_list_lock);
> >  	parent = __find_parent_device(dev);
> > -
> >  	if (!parent) {
> >  		mutex_unlock(&parent_list_lock);
> >  		return;
> >  	}
> > +	list_del(&parent->next);
> > +	mutex_unlock(&parent_list_lock);
> > +
> >  	dev_info(dev, "MDEV: Unregistering\n");
> >
> > -	list_del(&parent->next);
> > +	/* Publish that this mdev parent is unregistering. So any new
> > +	 * create/remove cannot start on this parent anymore by user.
> > +	 */
> 
> Comment style, we're not in netdev.
Yep. Will fix it.
> 
> > +	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.
> > +	 */
> >  	class_compat_remove_link(mdev_bus_compat_class, dev, NULL);
> >
> >  	device_for_each_child(dev, NULL, mdev_device_remove_cb);
> >
> >  	parent_remove_sysfs_files(parent);
> >
> > -	mutex_unlock(&parent_list_lock);
> >  	mdev_put_parent(parent);
> >  }
> >  EXPORT_SYMBOL(mdev_unregister_device);
> > @@ -278,14 +270,24 @@ static void mdev_device_release(struct device
> > *dev)  int mdev_device_create(struct kobject *kobj, struct device
> > *dev, uuid_le uuid)  {
> >  	int ret;
> > +	struct mdev_parent *valid_parent;
> >  	struct mdev_device *mdev, *tmp;
> >  	struct mdev_parent *parent;
> >  	struct mdev_type *type = to_mdev_type(kobj);
> > +	int srcu_idx;
> >
> >  	parent = mdev_get_parent(type->parent);
> >  	if (!parent)
> >  		return -EINVAL;
> >
> > +	srcu_idx = srcu_read_lock(&parent->unreg_srcu);
> > +	valid_parent = srcu_dereference(parent->self, &parent->unreg_srcu);
> > +	if (!valid_parent) {
> > +		/* parent is undergoing unregistration */
> > +		ret = -ENODEV;
> > +		goto mdev_fail;
> > +	}
> > +
> >  	mutex_lock(&mdev_list_lock);
> >
> >  	/* Check for duplicate */
> > @@ -310,68 +312,76 @@ int mdev_device_create(struct kobject *kobj,
> > struct device *dev, uuid_le uuid)
> >
> >  	mdev->parent = parent;
> >
> > +	device_initialize(&mdev->dev);
> >  	mdev->dev.parent  = dev;
> >  	mdev->dev.bus     = &mdev_bus_type;
> >  	mdev->dev.release = mdev_device_release;
> > +	mdev->dev.groups = type->parent->ops->mdev_attr_groups;
> >  	dev_set_name(&mdev->dev, "%pUl", uuid.b);
> >
> > -	ret = device_register(&mdev->dev);
> > +	ret = type->parent->ops->create(kobj, mdev);
> >  	if (ret)
> > -		goto mdev_fail;
> > +		goto create_fail;
> >
> > -	ret = mdev_device_create_ops(kobj, mdev);
> > +	ret = device_add(&mdev->dev);
> 
> Separating device_initialize() and device_add() also looks like a separate
> patch, then the srcu could be added at the end.  Thanks,
> 
> Alex

I saw little more core generated that way, but I think its fine.
Basically, create/remove callback sequencing that does the device_inititailze/add etc in one patch and 
User side race handling using srcu in another patch.
Sounds good?