On Wed, 30 Oct 2019 18:57:10 +0800
linmiaohe <linmiaohe@xxxxxxxxxx> wrote:
> From: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>
> The driver of the pci device may not equal to vfio_pci_driver,
> but we fetch vfio_device from pci_dev unconditionally in func
> vfio_pci_reflck_find. This overhead, such as the competition
> of vfio.group_lock, can be eliminated by check pci_dev_driver
> with vfio_pci_driver first.
>
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
> drivers/vfio/pci/vfio_pci.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 379a02c36e37..1e21970543a6 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -1466,15 +1466,14 @@ static int vfio_pci_reflck_find(struct pci_dev *pdev, void *data)
> struct vfio_device *device;
> struct vfio_pci_device *vdev;
>
> - device = vfio_device_get_from_dev(&pdev->dev);
> - if (!device)
> - return 0;
> -
> if (pci_dev_driver(pdev) != &vfio_pci_driver) {
> - vfio_device_put(device);
> return 0;
> }
>
> + device = vfio_device_get_from_dev(&pdev->dev);
> + if (!device)
> + return 0;
> +
> vdev = vfio_device_data(device);
>
> if (vdev->reflck) {
I believe this introduces a race. When we hold a reference to the vfio
device, an unbind from a vfio bus driver will be blocked in
vfio_del_group_dev(). Therefore if we test the driver is vfio-pci
while holding this reference, we know that it cannot be released and
the device_data is a valid vfio_pci_device. Testing the driver prior
to acquiring a vfio device reference is meaningless as we have no
guarantee that the driver has not changed by the time we acquire a
reference. Are you actually seeing contention here or was this a code
inspection optimization? Thanks,
Alex
