8266742: Check W^X state on possible safepoint

Reviewed-by: dholmes, gziemski

src/hotspot/os/bsd/globals_bsd.hpp

@@ -28,13 +28,17 @@
 //
 // Declare Bsd specific flags. They are not available on other platforms.
 //
-#define RUNTIME_OS_FLAGS(develop,     \
-                         develop_pd,  \
-                         product,     \
-                         product_pd,  \
-                         notproduct,  \
-                         range,       \
-                         constraint)
+#define RUNTIME_OS_FLAGS(develop,                                       \
+                         develop_pd,                                    \
+                         product,                                       \
+                         product_pd,                                    \
+                         notproduct,                                    \
+                         range,                                         \
+                         constraint)                                    \
+                                                                        \
+  AARCH64_ONLY(develop(bool, AssertWXAtThreadSync, false,                \
+          "Conservatively check W^X thread state at possible safepoint" \
+          "or handshake"))
 
 // end of RUNTIME_OS_FLAGS
 

src/hotspot/share/runtime/safepointMechanism.inline.hpp

@@ -65,6 +65,16 @@ bool SafepointMechanism::should_process(JavaThread* thread) {
 }
 
 void SafepointMechanism::process_if_requested(JavaThread* thread) {
+
+  // Macos/aarch64 should be in the right state for safepoint (e.g.
+  // deoptimization needs WXWrite).  Crashes caused by the wrong state rarely
+  // happens in practice, making such issues hard to find and reproduce.
+#if defined(ASSERT) && defined(__APPLE__) && defined(AARCH64)
+  if (AssertWXAtThreadSync) {
+    thread->assert_wx_state(WXWrite);
+  }
+#endif
+
   if (local_poll_armed(thread)) {
     process_if_requested_slow(thread);
   }

src/hotspot/share/runtime/thread.hpp

@@ -649,6 +649,10 @@ protected:
  public:
   void init_wx();
   WXMode enable_wx(WXMode new_state);
+
+  void assert_wx_state(WXMode expected) {
+    assert(_wx_state == expected, "wrong state");
+  }
 #endif // __APPLE__ && AARCH64
 };