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SubjectRe: [PATCH v3] vfio/mdev: Check globally for duplicate devices
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On 5/17/2018 1:39 PM, Cornelia Huck wrote:
> 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.
>

mdev_parent.lock is to serialize create and remove of that mdev device,
that handles race condition that Cornelia mentioned below.

> 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_put_parent(parent) missing before return.


>> mdev = kzalloc(sizeof(*mdev), GFP_KERNEL);
>> if (!mdev) {
>> - ret = -ENOMEM;
>> - goto create_err;
>> + mutex_unlock(&mdev_list_lock);
>> + return -ENOMEM;
>> }
>>

mdev_put_parent(parent) missing here again.

Thanks,
Kirti

>> 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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