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Subject: Re: [PATCH v2] vfio/mdev: Check globally for duplicate devices
To:     Alex Williamson <alex.williamson@redhat.com>
CC:     <kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <cohuck@redhat.com>
References: <20180516152228.12123.34883.stgit@gimli.home>
From:   Kirti Wankhede <kwankhede@nvidia.com>
Message-ID: <2c45cf24-f67d-bae6-59bf-aedb434ee843@nvidia.com>
Date:   Thu, 17 May 2018 01:01:40 +0530
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On 5/16/2018 8:53 PM, Alex Williamson 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.
> 
> Notably, mdev_parent.lock really only seems to be serializing device
> creation and removal per parent.  I'm not sure if this is necessary,
> mdev vendor drivers could easily provide this serialization if it
> is required, but a side-effect of holding the mdev_list_lock to
> protect the namespace is actually greater serialization across the
> create and remove paths,

Exactly for this reason more granular lock is used and that's the reason
mdev_parent.lock was introduced. Consider the max supported config for
vGPU: 8 GPUs in a system with 16 mdev devices on each GPUs, i.e. 128
mdev devices need to be created in a system, and this count will
increase in future, all mdev device creation/removal will get serialized
with this change.
I agree with your concern that if there are duplicates across parents,
its not caught earlier.

> so mdev_parent.lock is removed.  If we can
> show that vendor drivers handle the create/remove paths themselves,
> perhaps we can refine the locking granularity.
> 

Here lock is not for create/remove routines of vendor driver, its about
mdev device creation and device registration, which is a common code
path, and so is part of mdev core module.


> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
> 
> v2: Remove unnecessary ret init per Cornelia's review
> 
>  drivers/vfio/mdev/mdev_core.c    |   77 +++++++++-----------------------------
>  drivers/vfio/mdev/mdev_private.h |    1 
>  2 files changed, 19 insertions(+), 59 deletions(-)
> 
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index 126991046eb7..aaab3ef93e1c 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);
>  
> @@ -312,12 +283,14 @@ 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)) {
> +			ret = -EEXIST;
> +			goto create_err;
> +		}
>  	}
>
Is it possible to use mdev_list_lock for as minimal portion as possible?
By adding mdev device to mdev_list just after:
	memcpy(&mdev->uuid, &uuid, sizeof(uuid_le));
and then unlock mdev_list_lock, but at the same time all later error
cases need to be handled properly in this function.

Thanks,
Kirti


>  	mdev = kzalloc(sizeof(*mdev), GFP_KERNEL);
> @@ -354,9 +327,6 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
>  	mdev->type_kobj = kobj;
>  	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);
>  
> @@ -366,7 +336,7 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
>  	device_unregister(&mdev->dev);
>  
>  create_err:
> -	mutex_unlock(&parent->lock);
> +	mutex_unlock(&mdev_list_lock);
>  	mdev_put_parent(parent);
>  	return ret;
>  }
> @@ -382,6 +352,7 @@ int mdev_device_remove(struct device *dev, bool force_remove)
>  	mdev = to_mdev_device(dev);
>  
>  	mutex_lock(&mdev_list_lock);
> +
>  	list_for_each_entry(tmp, &mdev_list, next) {
>  		if (tmp == mdev) {
>  			found = true;
> @@ -389,35 +360,25 @@ int mdev_device_remove(struct device *dev, bool force_remove)
>  		}
>  	}
>  
> -	if (found)
> -		list_del(&mdev->next);
> -
> -	mutex_unlock(&mdev_list_lock);
> -
> -	if (!found)
> -		return -ENODEV;
> +	if (!found) {
> +		ret = -ENODEV;
> +		goto out;
> +	}
>  
>  	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);
> -
> -		return ret;
> -	}
> +	if (ret)
> +		goto out;
>  
> +	list_del(&mdev->next);
>  	mdev_remove_sysfs_files(dev, type);
>  	device_unregister(dev);
> -	mutex_unlock(&parent->lock);
>  	mdev_put_parent(parent);
> -
> -	return 0;
> +out:
> +	mutex_unlock(&mdev_list_lock);
> +	return ret;
>  }
>  
>  static int __init mdev_init(void)
> diff --git a/drivers/vfio/mdev/mdev_private.h b/drivers/vfio/mdev/mdev_private.h
> index a9cefd70a705..85bb268c91be 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;
>