diff --git a/src/arch/x86/isa/insts/general_purpose/semaphores.py b/src/arch/x86/isa/insts/general_purpose/semaphores.py
index de7c6ef631..aae67fac70 100644
--- a/src/arch/x86/isa/insts/general_purpose/semaphores.py
+++ b/src/arch/x86/isa/insts/general_purpose/semaphores.py
@@ -132,6 +132,7 @@ cmpxchg8bCode = '''
 def macroop CMPXCHG8B_%(suffix)s {
     .adjust_env clampOsz
     %(rdip)s
+    %(mfence)s
     lea t1, seg, %(sib)s, disp, dataSize=asz
     ldsplit%(l)s (t2, t3), seg, [1, t0, t1], disp=0
 
@@ -150,20 +151,25 @@ doneComparing:
 
     # Write to memory
     stsplit%(ul)s (t2, t3), seg, [1, t0, t1], disp=0
+    %(mfence)s
 };
 '''
 
 microcode += cmpxchg8bCode % {"rdip": "", "sib": "sib",
                               "l": "", "ul": "",
+                              "mfence": "",
                               "suffix": "M"}
 microcode += cmpxchg8bCode % {"rdip": "rdip t7", "sib": "riprel",
                               "l": "", "ul": "",
+                              "mfence": "",
                               "suffix": "P"}
 microcode += cmpxchg8bCode % {"rdip": "", "sib": "sib",
                               "l": "l", "ul": "ul",
+                              "mfence": "mfence",
                               "suffix": "LOCKED_M"}
 microcode += cmpxchg8bCode % {"rdip": "rdip t7", "sib": "riprel",
                               "l": "l", "ul": "ul",
+                              "mfence": "mfence",
                               "suffix": "LOCKED_P"}
 
 #let {{