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Date:   Thu, 17 May 2018 10:09:28 +0200
From:   Cornelia Huck <cohuck@redhat.com>
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     kwankhede@nvidia.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Dong Jia Shi <bjsdjshi@linux.ibm.com>,
        Halil Pasic <pasic@linux.ibm.com>
Subject: Re: [PATCH v3] vfio/mdev: Check globally for duplicate devices
Message-ID: <20180517100928.5949b11e.cohuck@redhat.com>
In-Reply-To: <20180517031746.32242.17768.stgit@gimli.home>
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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)
>