| Message ID | 1572433030-6267-1-git-send-email-linmiaohe@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | VFIO: PCI: eliminate unnecessary overhead in vfio_pci_reflck_find | expand |
On Wed, 30 Oct 2019 18:57:10 +0800 linmiaohe <linmiaohe@huawei.com> wrote: > From: Miaohe Lin <linmiaohe@huawei.com> > > 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@huawei.com> > --- > 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
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) {