rocr: Fix sem_post overflow errors

WaitSemaphore and PostSemaphore are used in the HybridMutex
implementation. If HybridMutex did not have to call WaitSemaphore when
acquired, then calling PostSemaphore would cause the internal count
inside sem_t to slowly grow to large values and eventually cause
overflow.

Change-Id: I173fc17c874b49926e56991405e9086ea8c138fc

runtime/hsa-runtime/core/util/lnx/os_linux.cpp

@@ -369,6 +369,14 @@ bool WaitSemaphore(Semaphore sem) {
 }
 
 void PostSemaphore(Semaphore sem) {
+  int waitval = 1;
+  if (sem_getvalue(*(sem_t**)&sem, &waitval))
+    assert(false && "Failed to get semaphore waiters");
+
+  /* sem_getvalue return <= 0 when there are threads blocked on sem_wait */
+  if (waitval > 0)
+    return;
+
   if (sem_post(*(sem_t**)&sem))
     assert(false && "Failed to post semaphore");
 }