diff --git a/sys/ia64/ia64/machdep.c b/sys/ia64/ia64/machdep.c
index fd5d781ca7eb..8bebde2d5cc2 100644
--- a/sys/ia64/ia64/machdep.c
+++ b/sys/ia64/ia64/machdep.c
@@ -442,7 +442,7 @@ cpu_switch(struct thread *old, struct thread *new, struct mtx *mtx)
 	if (PCPU_GET(fpcurthread) == old)
 		old->td_frame->tf_special.psr |= IA64_PSR_DFH;
 	if (!savectx(oldpcb)) {
-		old->td_lock = mtx;
+		atomic_store_rel_ptr(&old->td_lock, mtx);
 #if defined(SCHED_ULE) && defined(SMP)
 		/* td_lock is volatile */
 		while (new->td_lock == &blocked_lock)