Pastebin:Atomic

The question is, are atomic_1 and atomic_2 equivalent? Are they atomic? Are they thread-safe? (Continuing a discussion from #rosettacode)

Feel free to edit this to heck and back; If it needs to be linked to, link to a specific revision.

volatile LONG var;

class statmux {

 public:
 HANDLE hMux;
 statmux()
 {
   // Initialize the mutex
   hMux = CreateMutex(NULL, FALSE, NULL);
 }
 ~statmux()
 {
   // Free the mutex
   CloseHandle(hMux);
 }

} s;

void atomic_1(LONG newval) {

 // Lock the mutex
 DWORD res = WaitForSingleObject(s.hMux, INFINITE);

 // WAIT_OBJECT_0 indicates that we hold the mutex.  For these purposes, we don't care about any other scenario.
 if(WAIT_OBJECT_0 != res)
   return;

 // Replace the file-static variable value with the value we were passed.
 var = newval;

 // Release the mutex
 ReleaseMutex(s.hMux);

}

void atomic_2(LONG newval) {

 // InterlockedExchange sets up memory fences on either side of the load/store
 // Theoretically, no other thread will see a mixture of the old and new value (i.e. tearing)
 // Additionally, no other CPU will subsequently hold cached version of the old value.
 InterlockedExchange(&var, newval);

}