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Subject: Re: [PATCH v3] vfio/mdev: Check globally for duplicate devices
To:     Cornelia Huck <cohuck@redhat.com>,
        Alex Williamson <alex.williamson@redhat.com>
CC:     <kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Dong Jia Shi <bjsdjshi@linux.ibm.com>,
        Halil Pasic <pasic@linux.ibm.com>
References: <20180517031746.32242.17768.stgit@gimli.home>
 <20180517100928.5949b11e.cohuck@redhat.com>
From:   Kirti Wankhede <kwankhede@nvidia.com>
Message-ID: <50191254-4437-8ff3-06db-8d6b2df20b65@nvidia.com>
Date:   Thu, 17 May 2018 21:25:22 +0530
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In-Reply-To: <20180517100928.5949b11e.cohuck@redhat.com>
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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)
>>
>