diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 809fe3536301..4631cf99e714 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -605,7 +605,8 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 
 		/* Swap over from MISSING to IDENTITY if needed. */
 		if (p2m_top[topidx][mididx] == p2m_missing) {
-			p2m_top[topidx][mididx] = p2m_identity;
+			WARN_ON(cmpxchg(&p2m_top[topidx][mididx], p2m_missing,
+				p2m_identity) != p2m_missing);
 			return true;
 		}
 	}