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SubjectRe: [PATCH v3] vfio/mdev: Check globally for duplicate devices
On Wed, 16 May 2018 21:30:19 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> When we create an mdev device, we check for duplicates against the
> parent device and return -EEXIST if found, but the mdev device
> namespace is global since we'll link all devices from the bus. We do
> catch this later in sysfs_do_create_link_sd() to return -EEXIST, but
> with it comes a kernel warning and stack trace for trying to create
> duplicate sysfs links, which makes it an undesirable response.
>
> Therefore we should really be looking for duplicates across all mdev
> parent devices, or as implemented here, against our mdev device list.
> Using mdev_list to prevent duplicates means that we can remove
> mdev_parent.lock, but in order not to serialize mdev device creation
> and removal globally, we add mdev_device.active which allows UUIDs to
> be reserved such that we can drop the mdev_list_lock before the mdev
> device is fully in place.
>
> NB. there was never intended to be any serialization guarantee
> provided by the mdev core with respect to creation and removal of mdev
> devices, mdev_parent.lock provided this only as a side-effect of the
> implementation for locking the namespace per parent. That
> serialization is now removed.

This is probably fine; but I noted that documentation on the locking
conventions and serialization guarantees for mdev is a bit sparse, and
this topic also came up during the vfio-ap review.

We probably want to add some more concrete documentation; would the
kernel doc for the _ops or vfio-mediated-device.txt be a better place
for that?

[Dong Jia, Halil: Can you please take a look whether vfio-ccw is really
ok? I don't think we open up any new races, but I'd appreciate a second
or third opinion.]

>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>
> v3: Rework locking and add a field to mdev_device so we can track
> completed instances vs those added to reserve the namespace.
>
> drivers/vfio/mdev/mdev_core.c | 94 +++++++++++++-------------------------
> drivers/vfio/mdev/mdev_private.h | 2 -
> 2 files changed, 34 insertions(+), 62 deletions(-)
>
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index 126991046eb7..55ea9d34ec69 100644
> --- a/drivers/vfio/mdev/mdev_core.c
> +++ b/drivers/vfio/mdev/mdev_core.c
> @@ -66,34 +66,6 @@ uuid_le mdev_uuid(struct mdev_device *mdev)
> }
> EXPORT_SYMBOL(mdev_uuid);
>
> -static int _find_mdev_device(struct device *dev, void *data)
> -{
> - struct mdev_device *mdev;
> -
> - if (!dev_is_mdev(dev))
> - return 0;
> -
> - mdev = to_mdev_device(dev);
> -
> - if (uuid_le_cmp(mdev->uuid, *(uuid_le *)data) == 0)
> - return 1;
> -
> - return 0;
> -}
> -
> -static bool mdev_device_exist(struct mdev_parent *parent, uuid_le uuid)
> -{
> - struct device *dev;
> -
> - dev = device_find_child(parent->dev, &uuid, _find_mdev_device);
> - if (dev) {
> - put_device(dev);
> - return true;
> - }
> -
> - return false;
> -}
> -
> /* Should be called holding parent_list_lock */
> static struct mdev_parent *__find_parent_device(struct device *dev)
> {
> @@ -221,7 +193,6 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
> }
>
> kref_init(&parent->ref);
> - mutex_init(&parent->lock);
>
> parent->dev = dev;
> parent->ops = ops;
> @@ -304,7 +275,7 @@ 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_device *mdev;
> + struct mdev_device *mdev, *tmp;
> struct mdev_parent *parent;
> struct mdev_type *type = to_mdev_type(kobj);
>
> @@ -312,21 +283,26 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
> if (!parent)
> return -EINVAL;
>
> - mutex_lock(&parent->lock);
> + mutex_lock(&mdev_list_lock);
>
> /* Check for duplicate */
> - if (mdev_device_exist(parent, uuid)) {
> - ret = -EEXIST;
> - goto create_err;
> + list_for_each_entry(tmp, &mdev_list, next) {
> + if (!uuid_le_cmp(tmp->uuid, uuid)) {
> + mutex_unlock(&mdev_list_lock);
> + return -EEXIST;
> + }
> }
>
> mdev = kzalloc(sizeof(*mdev), GFP_KERNEL);
> if (!mdev) {
> - ret = -ENOMEM;
> - goto create_err;
> + mutex_unlock(&mdev_list_lock);
> + return -ENOMEM;
> }
>
> memcpy(&mdev->uuid, &uuid, sizeof(uuid_le));
> + list_add(&mdev->next, &mdev_list);
> + mutex_unlock(&mdev_list_lock);
> +
> mdev->parent = parent;
> kref_init(&mdev->ref);
>
> @@ -352,21 +328,18 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
> }
>
> mdev->type_kobj = kobj;
> + mdev->active = true;
> dev_dbg(&mdev->dev, "MDEV: created\n");
>
> - mutex_unlock(&parent->lock);
> -
> - mutex_lock(&mdev_list_lock);
> - list_add(&mdev->next, &mdev_list);
> - mutex_unlock(&mdev_list_lock);
> -
> - return ret;
> + return 0;
>
> create_failed:
> device_unregister(&mdev->dev);
>
> create_err:
> - mutex_unlock(&parent->lock);
> + mutex_lock(&mdev_list_lock);
> + list_del(&mdev->next);
> + mutex_unlock(&mdev_list_lock);
> mdev_put_parent(parent);
> return ret;
> }
> @@ -377,44 +350,43 @@ int mdev_device_remove(struct device *dev, bool force_remove)
> struct mdev_parent *parent;
> struct mdev_type *type;
> int ret;
> - bool found = false;
>
> mdev = to_mdev_device(dev);
>
> mutex_lock(&mdev_list_lock);
> list_for_each_entry(tmp, &mdev_list, next) {
> - if (tmp == mdev) {
> - found = true;
> + if (tmp == mdev)
> break;
> - }
> }
>
> - if (found)
> - list_del(&mdev->next);
> + if (tmp != mdev) {
> + mutex_unlock(&mdev_list_lock);
> + return -ENODEV;
> + }
>
> - mutex_unlock(&mdev_list_lock);
> + if (!mdev->active) {
> + mutex_unlock(&mdev_list_lock);
> + return -EAGAIN;
> + }

I'm not sure whether this is 100% watertight. Consider:

- device gets registered, we have added it to the list, made it visible
in sysfs and have added the remove attribute, but not yet the symlinks
- userspace can access the remove attribute and trigger removal
- we do an early exit here because not yet active
- ???

(If there's any problem, it's a very pathological case, and I don't
think anything really bad can happen. I just want to make sure we don't
miss anything.)

>
> - if (!found)
> - return -ENODEV;
> + mdev->active = false;
> + mutex_unlock(&mdev_list_lock);
>
> type = to_mdev_type(mdev->type_kobj);
> parent = mdev->parent;
> - mutex_lock(&parent->lock);
>
> ret = mdev_device_remove_ops(mdev, force_remove);
> if (ret) {
> - mutex_unlock(&parent->lock);
> -
> - mutex_lock(&mdev_list_lock);
> - list_add(&mdev->next, &mdev_list);
> - mutex_unlock(&mdev_list_lock);
> -
> + mdev->active = true;
> return ret;
> }
>
> + mutex_lock(&mdev_list_lock);
> + list_del(&mdev->next);
> + mutex_unlock(&mdev_list_lock);
> +
> mdev_remove_sysfs_files(dev, type);
> device_unregister(dev);
> - mutex_unlock(&parent->lock);
> mdev_put_parent(parent);
>
> return 0;
> diff --git a/drivers/vfio/mdev/mdev_private.h b/drivers/vfio/mdev/mdev_private.h
> index a9cefd70a705..b5819b7d7ef7 100644
> --- a/drivers/vfio/mdev/mdev_private.h
> +++ b/drivers/vfio/mdev/mdev_private.h
> @@ -20,7 +20,6 @@ struct mdev_parent {
> struct device *dev;
> const struct mdev_parent_ops *ops;
> struct kref ref;
> - struct mutex lock;
> struct list_head next;
> struct kset *mdev_types_kset;
> struct list_head type_list;
> @@ -34,6 +33,7 @@ struct mdev_device {
> struct kref ref;
> struct list_head next;
> struct kobject *type_kobj;
> + bool active;
> };
>
> #define to_mdev_device(dev) container_of(dev, struct mdev_device, dev)
>

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