linux-kernel - Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to migration driver

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Date:   Tue, 18 Oct 2022 15:06:43 +0800
From:   liulongfang <liulongfang@...wei.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     John Garry <john.garry@...wei.com>, <alex.williamson@...hat.com>,
        <shameerali.kolothum.thodi@...wei.com>, <cohuck@...hat.com>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>
Subject: Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to
 migration driver

On 2022/10/17 20:11, Jason Gunthorpe Wrote:
> On Mon, Oct 17, 2022 at 05:20:34PM +0800, liulongfang wrote:
>> On 2022/10/14 17:20, John Garry wrote:
>>> On 14/10/2022 03:57, Longfang Liu wrote:
>>>> +static void hisi_acc_vf_debugfs_init(struct hisi_acc_vf_core_device *hisi_acc_vdev)
>>>> +{
>>>> +    struct pci_dev *vf_pdev = hisi_acc_vdev->vf_dev;
>>>> +    struct device *dev = &vf_pdev->dev;
>>>> +    int ret;
>>>> +
>>>> +    if (!atomic_read(&hisi_acc_root_ref))
>>>> +        hisi_acc_debugfs_root = debugfs_create_dir("hisi_vfio_acc", NULL);
>>>> +    atomic_inc(&hisi_acc_root_ref);
>>>> +
>>>
>>> This looks totally racy, such that I wonder why even bother using an atomic for hisi_acc_root_ref.
>>
>>
>> When enabling VF, it is possible for multiple VMs to enable VF at the same time. The atomic variable
>> is used to ensure that only one "hisi_vfio_acc" is created. When other VFs are enabled,
>> it will not be created again, but will be used directly.
> 
> It is still completely racy. Use a lock
> 

Do you have any suggested solutions?

> Jason
> .
> 
Thanks,
Longfang