[PATCH 2/5] vfio/pci: Annotate lockless accesses to vdev->sriov_active

[David Matlack <dmatlack@google.com>]

Use READ_ONCE() and WRITE_ONCE() to annotate the read of
vdev->sriov_active in vfio_pci_set_power_state() and the lockless write
that clears it in vfio_pci_core_sriov_configure().

vfio_pci_core_sriov_configure() intentionally clears sriov_active
without holding memory_lock, so the write can race with the read in
vfio_pci_set_power_state() (which runs under memory_lock). The race
itself is benign, at worst it results in a spurious -EBUSY while the
device stays in D0, but the accesses should be marked to prevent the
compiler from tearing or otherwise optimizing them and to document the
intentional data race (e.g. for KCSAN).

The write that sets sriov_active is done under memory_lock and is
serialized against the lockless write by device_lock, so it does not
need to be annotated.

Fixes: 40ef3edf151e ("vfio/pci: Use a private flag to prevent power state change with VFs")
Signed-off-by: David Matlack <dmatlack@google.com>
---
 drivers/vfio/pci/vfio_pci_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 95aee6b7cbb3..9a62797e50bc 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -332,7 +332,7 @@ int vfio_pci_set_power_state(struct vfio_pci_core_device *vdev, pci_power_t stat
 	lockdep_assert_held_write(&vdev->memory_lock);
 
 	/* Prevent changing power state for PFs with VFs enabled */
-	if (state > PCI_D0 && vdev->sriov_active)
+	if (state > PCI_D0 && READ_ONCE(vdev->sriov_active))
 		return -EBUSY;
 
 	if (vdev->needs_pm_restore) {
@@ -2396,7 +2396,7 @@ int vfio_pci_core_sriov_configure(struct vfio_pci_core_device *vdev,
 	 * state transition would at most result in an -EBUSY, leaving the
 	 * device in PCI_D0.
 	 */
-	vdev->sriov_active = false;
+	WRITE_ONCE(vdev->sriov_active, false);
 
 	mutex_lock(&vfio_pci_sriov_pfs_mutex);
 	list_del_init(&vdev->sriov_pfs_item);
-- 
2.56.0.317.g454f8302e1-goog