Re: [Qemu-devel] [PATCH 1/3] Mediated device Core driver

[Kirti Wankhede]

On 6/29/2016 7:21 PM, Xiao Guangrong wrote:
> 
> 
> On 06/21/2016 12:31 AM, Kirti Wankhede wrote:
>> Design for Mediated Device Driver:
...
>> +static int mdev_add_attribute_group(struct device *dev,
>> +                    const struct attribute_group **groups)
>> +{
>> +    return sysfs_create_groups(&dev->kobj, groups);
>> +}
>> +
>> +static void mdev_remove_attribute_group(struct device *dev,
>> +                    const struct attribute_group **groups)
>> +{
>> +    sysfs_remove_groups(&dev->kobj, groups);
>> +}
>> +
> 
> better use device_add_groups() / device_remove_groups() instead?
> 

These are not exported from base module. They can't be used here.


>> +}
>> +
>> +static int mdev_device_create_ops(struct mdev_device *mdev, char
>> *mdev_params)
>> +{
>> +    struct parent_device *parent = mdev->parent;
>> +    int ret;
>> +
>> +    mutex_lock(&parent->ops_lock);
>> +    if (parent->ops->create) {
>> +        ret = parent->ops->create(mdev->dev.parent, mdev->uuid,
>> +                    mdev->instance, mdev_params);
> 
> I think it is better if we pass @mdev to this callback, then the parent
> driver
> can do its specified operations and associate it with the instance,
> e.g, via mdev->private.
> 

Yes, actually I was also thinking of changing it to

-       ret = parent->ops->create(mdev->dev.parent, mdev->uuid,
-                                 mdev->instance, mdev_params);
+       ret = parent->ops->create(mdev, mdev_params);


>> +int mdev_register_device(struct device *dev, const struct parent_ops
>> *ops)
>> +{
>> +    int ret = 0;
>> +    struct parent_device *parent;
>> +
>> +    if (!dev || !ops)
>> +        return -EINVAL;
>> +
>> +    mutex_lock(&parent_devices.list_lock);
>> +
>> +    /* Check for duplicate */
>> +    parent = find_parent_device(dev);
>> +    if (parent) {
>> +        ret = -EEXIST;
>> +        goto add_dev_err;
>> +    }
>> +
>> +    parent = kzalloc(sizeof(*parent), GFP_KERNEL);
>> +    if (!parent) {
>> +        ret = -ENOMEM;
>> +        goto add_dev_err;
>> +    }
>> +
>> +    kref_init(&parent->ref);
>> +    list_add(&parent->next, &parent_devices.dev_list);
>> +    mutex_unlock(&parent_devices.list_lock);
> 
> It is not safe as Alex's already pointed it out.
> 
>> +
>> +    parent->dev = dev;
>> +    parent->ops = ops;
>> +    mutex_init(&parent->ops_lock);
>> +    mutex_init(&parent->mdev_list_lock);
>> +    INIT_LIST_HEAD(&parent->mdev_list);
>> +    init_waitqueue_head(&parent->release_done);
> 
> And no lock to protect these operations.
> 

As I replied to Alex also, yes I'm fixing it.

>> +void mdev_unregister_device(struct device *dev)
>> +{
>> +    struct parent_device *parent;
>> +    struct mdev_device *mdev, *n;
>> +    int ret;
>> +
>> +    mutex_lock(&parent_devices.list_lock);
>> +    parent = find_parent_device(dev);
>> +
>> +    if (!parent) {
>> +        mutex_unlock(&parent_devices.list_lock);
>> +        return;
>> +    }
>> +    dev_info(dev, "MDEV: Unregistering\n");
>> +
>> +    /*
>> +     * Remove parent from the list and remove create and destroy sysfs
>> +     * files so that no new mediated device could be created for this
>> parent
>> +     */
>> +    list_del(&parent->next);
>> +    mdev_remove_sysfs_files(dev);
>> +    mutex_unlock(&parent_devices.list_lock);
>> +
> 
> find_parent_device() does not increase the refcount of the parent-device,
> after releasing the lock, is it still safe to use the device?
> 

Yes. In mdev_register_device(), kref_init() initialises refcount to 1
and then when mdev child is created refcount is incremented and on child
mdev destroys refcount is decremented. So when all child mdev are
destroyed, refcount will still be 1 until mdev_unregister_device() is
called. So when no mdev device is created, mdev_register_device() hold
parent's refcount and released from mdev_unregister_device().


>> +    mutex_lock(&parent->ops_lock);
>> +    mdev_remove_attribute_group(dev,
>> +                    parent->ops->dev_attr_groups);
> 
> Why mdev_remove_sysfs_files() and mdev_remove_attribute_group()
> are protected by different locks?
>

As mentioned in reply to Alex on another thread, removing these locks.

>> +
>> +int mdev_device_destroy(struct device *dev, uuid_le uuid, uint32_t
>> instance)
>> +{
>> +    struct mdev_device *mdev;
>> +    struct parent_device *parent;
>> +    int ret;
>> +
>> +    parent = mdev_get_parent_by_dev(dev);
>> +    if (!parent) {
>> +        ret = -EINVAL;
>> +        goto destroy_err;
>> +    }
>> +
>> +    mdev = find_mdev_device(parent, uuid, instance);
>> +    if (!mdev) {
>> +        ret = -EINVAL;
>> +        goto destroy_err;
>> +    }
>> +
>> +    ret = mdev_device_destroy_ops(mdev, false);
>> +    if (ret)
>> +        goto destroy_err;
> 
> find_mdev_device() does not hold the refcount of mdev, is it safe?
> 

Yes, this function is just to check duplicate entry or existence of mdev
device.

>> +
>> +    mdev_put_parent(parent);
>> +
> 
> The refcount of parent-device is released, you can not continue to
> use it.
> 

Removing these locks.

>> +    mutex_lock(&parent->mdev_list_lock);
>> +    list_del(&mdev->next);
>> +    mutex_unlock(&parent->mdev_list_lock);
>> +
>> +    mdev_put_device(mdev);
>> +    return ret;
>> +
>> +destroy_err:
>> +    mdev_put_parent(parent);
>> +    return ret;
>> +}
>> +
>> +
>> +static struct class mdev_class = {
>> +    .name        = MDEV_CLASS_NAME,
>> +    .owner        = THIS_MODULE,
>> +    .class_attrs    = mdev_class_attrs,
> 
> These interfaces, start and shutdown, are based on UUID, how
> about if we want to operate on the specified instance?
> 

Do you mean hot-plug a device?

>> +};
>> +
>> +static int __init mdev_init(void)
>> +{
>> +    int ret;
>> +
>> +    mutex_init(&parent_devices.list_lock);
>> +    INIT_LIST_HEAD(&parent_devices.dev_list);
>> +
>> +    ret = class_register(&mdev_class);
>> +    if (ret) {
>> +        pr_err("Failed to register mdev class\n");
>> +        return ret;
>> +    }
>> +
>> +    ret = mdev_bus_register();
>> +    if (ret) {
>> +        pr_err("Failed to register mdev bus\n");
>> +        class_unregister(&mdev_class);
>> +        return ret;
>> +    }
>> +
>> +    return ret;
>> +}
>> +
>> +static void __exit mdev_exit(void)
>> +{
>> +    mdev_bus_unregister();
>> +    class_unregister(&mdev_class);
>> +}
> 
> Hmm, how to prevent if there are parent-devices existing
> when the module is being unloaded?
>

If parent device exits that means that other module is using
mdev_register_device()/mdev_unregister_device() from their module.
'rmmod mdev' would fail until that module is unloaded.

# rmmod mdev
rmmod: ERROR: Module mdev is in use by: nvidia



>> +
>> +static int uuid_parse(const char *str, uuid_le *uuid)
>> +{
>> +    int i;
>> +
>> +    if (strlen(str) < UUID_CHAR_LENGTH)
>> +        return -EINVAL;
>> +
>> +    for (i = 0; i < UUID_BYTE_LENGTH; i++) {
>> +        if (!isxdigit(str[0]) || !isxdigit(str[1])) {
>> +            pr_err("%s err", __func__);
>> +            return -EINVAL;
>> +        }
>> +
>> +        uuid->b[i] = (hex_to_bin(str[0]) << 4) | hex_to_bin(str[1]);
>> +        str += 2;
>> +        if (is_uuid_sep(*str))
>> +            str++;
>> +    }
>> +
>> +    return 0;
>> +}
>> +
> 
> Can we use uuid_le_to_bin()?

I couldn't find this in kernel?

Thanks,
Kirti