Re: [PATCH v9 09/22] s390: vfio-ap: register matrix device with VFIO mdev framework

Tony Krowiak
2018-08-17T19:02:02Z

On 08/17/2018 04:43 AM, Cornelia Huck wrote:
> On Thu, 16 Aug 2018 12:24:16 -0400
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> On 08/14/2018 07:19 AM, Cornelia Huck wrote:
>>> On Mon, 13 Aug 2018 17:48:06 -0400
>>> Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:
>>>> +static int vfio_ap_mdev_create(struct kobject *kobj, struct mdev_device *mdev)
>>>> +{
>>>> + struct ap_matrix_mdev *matrix_mdev;
>>>> +
>>>> + matrix_mdev = kzalloc(sizeof(*matrix_mdev), GFP_KERNEL);
>>>> + if (!matrix_mdev)
>>>> + return -ENOMEM;
>>>> +
>>>> + matrix_mdev->name = dev_name(mdev_dev(mdev));
>>>> + vfio_ap_matrix_init(&matrix_dev.info, &matrix_mdev->matrix);
>>>> + mdev_set_drvdata(mdev, matrix_mdev);
>>>> +
>>>> + if (atomic_dec_if_positive(&matrix_dev.available_instances) < 0) {
>>>> + kfree(matrix_mdev);
>>>> + return -EPERM;
>>>> + }
>>> Maybe move this check to the top of the function?
>> Please ignore my previous response to your comment. I can't move the call to
>> atomic_dec_if_positive() to the top of the function because it
>> decrements the
>> available_instances and if the kzalloc() of matrix_mdev fails, then the
>> value
>> would have to then be incremented to remain valid. What I can do is this:
>>
>> 1. Check the value of available_instances using atomic_read() at the top of
>> the function and if it is zero, return an error.
>>
>> 2. Replace the call to atomic_dec_if_positive() with a call to atomic_dec()
>> to decrement the available_instances.
>>
>> I agree that it makes sense to return before attempting to allocate the
>> matrix_mdev if available_instances is zero.
> Wouldn't that be racy, though?
>
> I don't think re-incrementing the counter is too bad, and it's
> certainly better than going through allocation/freeing of structures.

I'll make it happen.

>
>>>
>>>> +
>>>> + mutex_lock(&matrix_dev.lock);
>>>> + list_add(&matrix_mdev->list, &matrix_dev.mdev_list);
>>>> + mutex_unlock(&matrix_dev.lock);
>>>> +
>>>> + return 0;
>>>> +}
>>