[RFC PATCH] vfio-pci: avoid deadlock between unbind and VFIO_DEVICE_RESET

When we unbind vfio-pci from a device, while running a guest, we might
have a deadlock when such a guest reboots.

Unbind takes device_lock at device_release_driver, and waits for
release_q at vfio_del_group_dev.

release_q will only be woken up when all references to vfio_device are
gone, and that includes open file descriptors, like the ones a guest
on qemu will hold.

If you try to reboot the guest, it will call VFIO_DEVICE_RESET, which
calls pci_reset_function, which now grabs the device_lock, and we are
deadlocked.

Using device_trylock allow us to handle the case when the lock is
already taken, and avoid this situation.

Signed-off-by: Thadeu Lima de Souza Cascardo <redacted>
---

Not tested yet, but I would like some comments now, like would it be
better to have a pci_try_reset_function, or do trylock on
pci_reset_function itself?

---
 drivers/vfio/pci/vfio_pci.c |   14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 3b76dc8..d1d2242 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c

@@ -513,8 +513,18 @@ static long vfio_pci_ioctl(void *device_data,
 		return ret;
 
 	} else if (cmd == VFIO_DEVICE_RESET) {
-		return vdev->reset_works ?
-			pci_reset_function(vdev->pdev) : -EINVAL;
+		struct pci_dev *pdev = vdev->pdev;
+		int ret = -EBUSY;
+		if (!vdev->reset_works)
+			return -EINVAL;
+		if (pci_cfg_access_trylock(pdev)) {
+			if (device_trylock(&pdev->dev)) {
+				ret = __pci_reset_function_locked(pdev);
+				device_unlock(&pdev->dev);
+			}
+			pci_cfg_access_unlock(pdev);
+		}
+		return ret;
 
 	} else if (cmd == VFIO_DEVICE_GET_PCI_HOT_RESET_INFO) {
 		struct vfio_pci_hot_reset_info hdr;

-- 
1.7.1