Index: sys/arm64/arm64/swtch.S
===================================================================
--- sys/arm64/arm64/swtch.S
+++ sys/arm64/arm64/swtch.S
@@ -41,6 +41,8 @@
  * void cpu_throw(struct thread *old, struct thread *new)
  */
 ENTRY(cpu_throw)
+	clrex
+
 #ifdef VFP
 	/* Backup the new thread pointer around a call to C code */
 	mov	x19, x1
@@ -98,6 +100,8 @@
  * x3 to x7, x16 and x17 are caller saved
  */
 ENTRY(cpu_switch)
+	clrex
+
 	/* Store the new curthread */
 	str	x1, [x18, #PC_CURTHREAD]
 	/* And the new pcb */
Index: sys/arm64/arm64/trap.c
===================================================================
--- sys/arm64/arm64/trap.c
+++ sys/arm64/arm64/trap.c
@@ -156,5 +156,7 @@
 	int error, sig, ucode;
 
+	clrex();
+
 #ifdef KDB
 	if (kdb_active) {
 		kdb_reenter();
Index: sys/arm64/include/cpufunc.h
===================================================================
--- sys/arm64/include/cpufunc.h
+++ sys/arm64/include/cpufunc.h
@@ -108,6 +108,17 @@
 	return (mpidr);
 }
 
+static __inline void
+clrex(void)
+{
+
+	/*
+	 * Ensure compiler barrier, otherwise the monitor clear might
+	 * occur too late for us ?
+	 */
+	__asm __volatile("clrex" : : : "memory");
+}
+
 #define	cpu_nullop()			arm64_nullop()
 #define	cpufunc_nullop()		arm64_nullop()
 #define	cpu_setttb(a)			arm64_setttb(a)