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

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Message-ID: <Y01GdiFbt2J9La2Y@nvidia.com>
Date:   Mon, 17 Oct 2022 09:11:34 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     liulongfang <liulongfang@...wei.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 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

Jason