[libvirt PATCH v5 27/30] nodedev: fix hang when destroying an mdev in use

[Jonathon Jongsma]

Calling `mdevctl stop` for a mediated device that is in use by an active
domain will block until that vm exits (or the vm closes the device).
Since the nodedev driver cannot query the hypervisor driver to see
whether any active domains are using the device, we resort to a
workaround that relies on the fact that a vfio group can only be opened
by one user at a time. If we get an EBUSY error when attempting to open
the group file, we assume the device is in use and refuse to try to
destroy that device.

Signed-off-by: Jonathon Jongsma <jjongsma at redhat.com>
Reviewed-by: Erik Skultety <eskultet at redhat.com>
---
 src/node_device/node_device_driver.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/node_device/node_device_driver.c b/src/node_device/node_device_driver.c
index a7ae3d5254..e702b30e54 100644
--- a/src/node_device/node_device_driver.c
+++ b/src/node_device/node_device_driver.c
@@ -1177,8 +1177,27 @@ nodeDeviceDestroy(virNodeDevicePtr device)
 
         ret = 0;
     } else if (nodeDeviceHasCapability(def, VIR_NODE_DEV_CAP_MDEV)) {
+        /* If this mediated device is in use by a vm, attempting to stop it
+         * will block until the vm closes the device. The nodedev driver
+         * cannot query the hypervisor driver to determine whether the device
+         * is in use by any active domains, since that would introduce circular
+         * dependencies between daemons and add a risk of deadlocks. So we need
+         * to resort to a workaround.  vfio only allows the group for a device
+         * to be opened by one user at a time. So if we get EBUSY when opening
+         * the group, we infer that the device is in use and therefore we
+         * shouldn't try to remove the device. */
+        g_autofree char *vfiogroup =
+            virMediatedDeviceGetIOMMUGroupDev(def->caps->data.mdev.uuid);
+        VIR_AUTOCLOSE fd = open(vfiogroup, O_RDONLY);
         g_autofree char *errmsg = NULL;
 
+        if (fd < 0 && errno == EBUSY) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("Unable to destroy '%s': device in use"),
+                           def->name);
+            goto cleanup;
+        }
+
         if (virMdevctlStop(def, &errmsg) < 0) {
             virReportError(VIR_ERR_INTERNAL_ERROR,
                            _("Unable to destroy '%s': %s"), def->name,
-- 
2.26.2