diff --git a/src/kernel/libroot/os/arch/x86/atomic.S b/src/kernel/libroot/os/arch/x86/atomic.S
index f3b3a6b03b..069104921d 100644
--- a/src/kernel/libroot/os/arch/x86/atomic.S
+++ b/src/kernel/libroot/os/arch/x86/atomic.S
@@ -51,6 +51,7 @@ _atomic_or1:
 FUNCTION(atomic_set):
 	movl	4(%esp),%edx
 	movl	8(%esp),%eax
+	lock
 	xchg	%eax,(%edx)
 	ret