src/hotspot/share/compiler/compileBroker.cpp

rev 54099 : 8219586: CodeHeap State Analytics processes dead nmethods
Reviewed-by: thartmann, eosterlund

@@ -2772,62 +2772,82 @@
   if (aggregate) {
     print_info(out);
   }
 
   // We hold the CodeHeapStateAnalytics_lock all the time, from here until we leave this function.
-  // That prevents another thread from destroying our view on the CodeHeap.
+  // That prevents other threads from destroying (making inconsistent) our view on the CodeHeap.
   // When we request individual parts of the analysis via the jcmd interface, it is possible
   // that in between another thread (another jcmd user or the vm running into CodeCache OOM)
-  // updated the aggregated data. That's a tolerable tradeoff because we can't hold a lock
-  // across user interaction.
-  // Acquire this lock before acquiring the CodeCache_lock.
-  // CodeHeapStateAnalytics_lock could be held by a concurrent thread for a long time,
-  // leading to an unnecessarily long hold time of the CodeCache_lock.
+  // updated the aggregated data. We will then see a modified, but again consistent, view
+  // on the CodeHeap. That's a tolerable tradeoff we have to accept because we can't hold
+  // a lock across user interaction.
+
+  // We should definitely acquire this lock before acquiring Compile_lock and CodeCache_lock.
+  // CodeHeapStateAnalytics_lock may be held by a concurrent thread for a long time,
+  // leading to an unnecessarily long hold time of the other locks we acquired before.
   ts.update(); // record starting point
-  MutexLockerEx mu1(CodeHeapStateAnalytics_lock, Mutex::_no_safepoint_check_flag);
+  MutexLockerEx mu0(CodeHeapStateAnalytics_lock);
   out->print_cr("\n__ CodeHeapStateAnalytics lock wait took %10.3f seconds _________\n", ts.seconds());
 
-  // If we serve an "allFun" call, it is beneficial to hold the CodeCache_lock
-  // for the entire duration of aggregation and printing. That makes sure
-  // we see a consistent picture and do not run into issues caused by
-  // the CodeHeap being altered concurrently.
-  Monitor* global_lock   = allFun ? CodeCache_lock : NULL;
-  Monitor* function_lock = allFun ? NULL : CodeCache_lock;
+  // Holding the CodeCache_lock protects from concurrent alterations of the CodeCache.
+  // Unfortunately, such protection is not sufficient:
+  // When a new nmethod is created via ciEnv::register_method(), the
+  // Compile_lock is taken first. After some initializations,
+  // nmethod::new_nmethod() takes over, grabbing the CodeCache_lock
+  // immediately (after finalizing the oop references). To lock out concurrent
+  // modifiers, we have to grab both locks as well in the described sequence.
+  //
+  // If we serve an "allFun" call, it is beneficial to hold CodeCache_lock and Compile_lock
+  // for the entire duration of aggregation and printing. That makes sure we see
+  // a consistent picture and do not run into issues caused by concurrent alterations.
+  bool should_take_Compile_lock   = !SafepointSynchronize::is_at_safepoint() &&
+                                    !Compile_lock->owned_by_self();
+  bool should_take_CodeCache_lock = !SafepointSynchronize::is_at_safepoint() &&
+                                    !CodeCache_lock->owned_by_self();
+  Mutex*   global_lock_1   = allFun ? (should_take_Compile_lock   ? Compile_lock   : NULL) : NULL;
+  Monitor* global_lock_2   = allFun ? (should_take_CodeCache_lock ? CodeCache_lock : NULL) : NULL;
+  Mutex*   function_lock_1 = allFun ? NULL : (should_take_Compile_lock   ? Compile_lock    : NULL);
+  Monitor* function_lock_2 = allFun ? NULL : (should_take_CodeCache_lock ? CodeCache_lock  : NULL);
   ts_global.update(); // record starting point
-  MutexLockerEx mu2(global_lock, Mutex::_no_safepoint_check_flag);
-  if (global_lock != NULL) {
-    out->print_cr("\n__ CodeCache (global) lock wait took %10.3f seconds _________\n", ts_global.seconds());
+  MutexLockerEx mu1(global_lock_1);
+  MutexLockerEx mu2(global_lock_2, Mutex::_no_safepoint_check_flag);
+  if ((global_lock_1 != NULL) || (global_lock_2 != NULL)) {
+    out->print_cr("\n__ Compile & CodeCache (global) lock wait took %10.3f seconds _________\n", ts_global.seconds());
     ts_global.update(); // record starting point
   }
 
   if (aggregate) {
     ts.update(); // record starting point
-    MutexLockerEx mu3(function_lock, Mutex::_no_safepoint_check_flag);
-    if (function_lock != NULL) {
-      out->print_cr("\n__ CodeCache (function) lock wait took %10.3f seconds _________\n", ts.seconds());
+    MutexLockerEx mu11(function_lock_1);
+    MutexLockerEx mu22(function_lock_2, Mutex::_no_safepoint_check_flag);
+    if ((function_lock_1 != NULL) || (function_lock_1 != NULL)) {
+      out->print_cr("\n__ Compile & CodeCache (function) lock wait took %10.3f seconds _________\n", ts.seconds());
     }
 
     ts.update(); // record starting point
     CodeCache::aggregate(out, granularity);
-    if (function_lock != NULL) {
-      out->print_cr("\n__ CodeCache (function) lock hold took %10.3f seconds _________\n", ts.seconds());
+    if ((function_lock_1 != NULL) || (function_lock_1 != NULL)) {
+      out->print_cr("\n__ Compile & CodeCache (function) lock hold took %10.3f seconds _________\n", ts.seconds());
     }
   }
 
   if (usedSpace) CodeCache::print_usedSpace(out);
   if (freeSpace) CodeCache::print_freeSpace(out);
   if (methodCount) CodeCache::print_count(out);
   if (methodSpace) CodeCache::print_space(out);
   if (methodAge) CodeCache::print_age(out);
   if (methodNames) {
-    // print_names() has shown to be sensitive to concurrent CodeHeap modifications.
-    // Therefore, request  the CodeCache_lock before calling...
-    MutexLockerEx mu3(function_lock, Mutex::_no_safepoint_check_flag);
+    if (allFun) {
+      // print_names() can only be used safely if the locks have been continuously held
+      // since aggregation begin. That is true only for function "all".
     CodeCache::print_names(out);
+    } else {
+      out->print_cr("\nCodeHeapStateAnalytics: Function 'MethodNames' is only available as part of function 'all'");
+    }
   }
   if (discard) CodeCache::discard(out);
 
-  if (global_lock != NULL) {
-    out->print_cr("\n__ CodeCache (global) lock hold took %10.3f seconds _________\n", ts_global.seconds());
+  if ((global_lock_1 != NULL) || (global_lock_2 != NULL)) {
+    out->print_cr("\n__ Compile & CodeCache (global) lock hold took %10.3f seconds _________\n", ts_global.seconds());
   }
   out->print_cr("\n__ CodeHeapStateAnalytics total duration %10.3f seconds _________\n", ts_total.seconds());
 }