Index: sys/arch/xen/xen/clock.c
===================================================================
RCS file: /cvsroot/src/sys/arch/xen/xen/clock.c,v
retrieving revision 1.64
diff -p -u -r1.64 clock.c
--- sys/arch/xen/xen/clock.c	12 Jun 2016 09:08:09 -0000	1.64
+++ sys/arch/xen/xen/clock.c	28 Oct 2017 18:09:11 -0000
@@ -118,22 +118,29 @@ get_time_values_from_xen(struct cpu_info
 	KASSERT(mutex_owned(&tmutex));
 
 	do {
-		shadow->time_version = t->version;
+		if (1 & (shadow->time_version = t->version)) {
+			/* update in progress */
+			SPINLOCK_BACKOFF_HOOK();
+			continue;
+		}
 		xen_rmb();
 		shadow->tsc_stamp = t->tsc_timestamp;
 		shadow->system_time = t->system_time;
 		shadow->freq_mul = t->tsc_to_system_mul;
 		shadow->freq_shift = t->tsc_shift;
 		xen_rmb();
-	} while ((t->version & 1) || (shadow->time_version != t->version));
+	} while (shadow->time_version != t->version);
 	do {
-		tversion = HYPERVISOR_shared_info->wc_version;
+		if (1 & (tversion = HYPERVISOR_shared_info->wc_version)) {
+			/* update in progress */
+			SPINLOCK_BACKOFF_HOOK();
+			continue;
+		}
 		xen_rmb();
 		shadow->ts.tv_sec = HYPERVISOR_shared_info->wc_sec;
 		shadow->ts.tv_nsec = HYPERVISOR_shared_info->wc_nsec;
 		xen_rmb();
-	} while ((HYPERVISOR_shared_info->wc_version & 1) ||
-	    (tversion != HYPERVISOR_shared_info->wc_version));
+	} while (tversion != HYPERVISOR_shared_info->wc_version);
 }
 
 /*