8074553: Crash with assert(!is_unloaded()) failed: should not call follow on unloaded nmethod

Sweeper thread needs to override nmethods_do() to prevent G1 from unloading _scanned_nmethod.

Reviewed-by: kvn, mgerdin

hotspot/src/share/vm/runtime/thread.cpp

@@ -791,10 +791,6 @@ void Thread::oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobClosure* cf) {
   handle_area()->oops_do(f);
 }
 
-void Thread::nmethods_do(CodeBlobClosure* cf) {
-  // no nmethods in a generic thread...
-}
-
 void Thread::metadata_handles_do(void f(Metadata*)) {
   // Only walk the Handles in Thread.
   if (metadata_handles() != NULL) {
@@ -2827,8 +2823,6 @@ void JavaThread::oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobClosure* cf)
 }
 
 void JavaThread::nmethods_do(CodeBlobClosure* cf) {
-  Thread::nmethods_do(cf);  // (super method is a no-op)
-
   assert((!has_last_Java_frame() && java_call_counter() == 0) ||
          (has_last_Java_frame() && java_call_counter() > 0), "wrong java_sp info!");
 
@@ -3301,6 +3295,7 @@ CodeCacheSweeperThread::CodeCacheSweeperThread()
 : JavaThread(&sweeper_thread_entry) {
   _scanned_nmethod = NULL;
 }
+
 void CodeCacheSweeperThread::oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobClosure* cf) {
   JavaThread::oops_do(f, cld_f, cf);
   if (_scanned_nmethod != NULL && cf != NULL) {
@@ -3311,6 +3306,16 @@ void CodeCacheSweeperThread::oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobC
   }
 }
 
+void CodeCacheSweeperThread::nmethods_do(CodeBlobClosure* cf) {
+  JavaThread::nmethods_do(cf);
+  if (_scanned_nmethod != NULL && cf != NULL) {
+    // Safepoints can occur when the sweeper is scanning an nmethod so
+    // process it here to make sure it isn't unloaded in the middle of
+    // a scan.
+    cf->do_code_blob(_scanned_nmethod);
+  }
+}
+
 
 // ======= Threads ========
 
@@ -4348,9 +4353,13 @@ void Threads::create_thread_roots_marking_tasks(GCTaskQueue* q) {
 
 void Threads::nmethods_do(CodeBlobClosure* cf) {
   ALL_JAVA_THREADS(p) {
+    // This is used by the code cache sweeper to mark nmethods that are active
+    // on the stack of a Java thread. Ignore the sweeper thread itself to avoid
+    // marking CodeCacheSweeperThread::_scanned_nmethod as active.
+    if(!p->is_Code_cache_sweeper_thread()) {
       p->nmethods_do(cf);
     }
-  VMThread::vm_thread()->nmethods_do(cf);
+  }
 }
 
 void Threads::metadata_do(void f(Metadata*)) {

hotspot/src/share/vm/runtime/thread.hpp

@@ -509,9 +509,6 @@ class Thread: public ThreadShadow {
     }
   }
 
-  // Sweeper support
-  void nmethods_do(CodeBlobClosure* cf);
-
   // jvmtiRedefineClasses support
   void metadata_handles_do(void f(Metadata*));
 
@@ -1649,7 +1646,7 @@ class JavaThread: public Thread {
   void oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobClosure* cf);
 
   // Sweeper operations
-  void nmethods_do(CodeBlobClosure* cf);
+  virtual void nmethods_do(CodeBlobClosure* cf);
 
   // RedefineClasses Support
   void metadata_do(void f(Metadata*));
@@ -1997,10 +1994,10 @@ class CodeCacheSweeperThread : public JavaThread {
   bool is_hidden_from_external_view() const { return true; }
 
   bool is_Code_cache_sweeper_thread() const { return true; }
-  // GC support
+
-  // Apply "f->do_oop" to all root oops in "this".
+  // Prevent GC from unloading _scanned_nmethod
-  // Apply "cf->do_code_blob" (if !NULL) to all code blobs active in frames
   void oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobClosure* cf);
+  void nmethods_do(CodeBlobClosure* cf);
 };
 
 // A thread used for Compilation.