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Date: Thu, 17 May 2018 01:01:40 +0530
From: Kirti Wankhede <kwankhede@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<cohuck@...hat.com>
Subject: Re: [PATCH v2] vfio/mdev: Check globally for duplicate devices
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@...hat.com>
> Signed-off-by: Alex Williamson <alex.williamson@...hat.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;
>
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