[nrf fromtree] ipc: Drain pending work items before deregistering end…

…point

The work item will attempt to dereference pointers that have been nulled
by the backend.

To avoid that, wait until all items currently on the queue have been
processed.

The symptom is a busfault on ARM, and is "fixed" by adding a
`k_msleep(1)` right before `ipc_service_deregister_endpoint()`. This
will in effect do the same thing as this patch, and allow the scheduler
to run the work item on the ipc workqueue.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
(cherry picked from commit f24a01e)

subsys/ipc/ipc_service/backends/ipc_rpmsg_static_vrings.c

@@ -456,6 +456,7 @@ static int deregister_ept(const struct device *instance, void *token)
 {
 	struct backend_data_t *data = instance->data;
 	struct ipc_rpmsg_ept *rpmsg_ept;
+	static struct k_work_sync sync;
 
 	/* Instance is not ready */
 	if (atomic_get(&data->state) != STATE_INITED) {
@@ -469,6 +470,13 @@ static int deregister_ept(const struct device *instance, void *token)
 		return -ENOENT;
 	}
 
+	/* Drain pending work items before tearing down channel.
+	 *
+	 * Note: `k_work_flush` Faults on Cortex-M33 with "illegal use of EPSR"
+	 * if `sync` is not declared static.
+	 */
+	k_work_flush(&data->mbox_work, &sync);
+
 	rpmsg_destroy_ept(&rpmsg_ept->ep);
 
 	memset(rpmsg_ept, 0, sizeof(struct ipc_rpmsg_ept));