Add missing storeload memory fences

There is no synchronize with relationship between the monitor micro-
lock and the onDeck microlock, so it is possible for an onDeck.load to
move above a contendersList.store, or a contendersList.load to move
above an ondeck.store.

To fix this issue a full memory fence (mm_mfence on x86) is needed
after the last store in the contendersList and onDeck critical regions.

Change-Id: I5beb7dfe0d21010c5bf00cd65d59b9c7af58e919


[ROCm/clr commit: f10435a1ef]

projects/clr/rocclr/thread/monitor.cpp

@@ -205,6 +205,10 @@ void Monitor::finishUnlock() {
       return;
     }
 
+    // A StoreLoad barrier is required to make sure the onDeck_ store is published before
+    // the contendersList_ micro-lock check.
+    std::atomic_thread_fence(std::memory_order_seq_cst);
+
     // We do not have an on-deck thread (semaphore == NULL). Return if
     // the contention list is empty or if the lock got acquired again.
     head = contendersList_;

projects/clr/rocclr/thread/monitor.hpp

@@ -228,6 +228,11 @@ inline void Monitor::unlock() {
   while (!contendersList_.compare_exchange_weak(ptr, ptr & ~kLockBit, std::memory_order_acq_rel,
                                                 std::memory_order_acquire))
     ;
+
+  // A StoreLoad barrier is required to make sure future loads do not happen before the
+  // contendersList_ store is published.
+  std::atomic_thread_fence(std::memory_order_seq_cst);
+
   //
   // We succeeded the CAS from locked to unlocked.
   // This is the end of the critical region.