From 5ffb98df92a5bfb878ecde00551f2b951c51aa45 Mon Sep 17 00:00:00 2001
From: Andrew McCreight <amccreight@mozilla.com>
Date: Mon, 13 Jun 2011 13:24:23 -0700
Subject: [PATCH] Bug 663532 - Run GC from cycle collector in main thread, not
 CC thread. r=bent

---
 xpcom/base/nsCycleCollector.cpp | 66 ++++++++++++++++++---------------
 1 file changed, 36 insertions(+), 30 deletions(-)

diff --git a/xpcom/base/nsCycleCollector.cpp b/xpcom/base/nsCycleCollector.cpp
index 3e2897b4c3b..317ef6f7b8a 100644
--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -1047,12 +1047,12 @@ struct nsCycleCollector
                      nsICycleCollectorListener *aListener);
 
     // Prepare for and cleanup after one or more collection(s).
-    PRBool PrepareForCollection(nsTPtrArray<PtrInfo> *aWhiteNodes);
+    PRBool PrepareForCollection(nsTPtrArray<PtrInfo> *aWhiteNodes,
+                                PRBool aForceGC);
     void CleanupAfterCollection();
 
     // Start and finish an individual collection.
-    PRBool BeginCollection(PRBool aForceGC,
-                           nsICycleCollectorListener *aListener);
+    PRBool BeginCollection(nsICycleCollectorListener *aListener);
     PRBool FinishCollection();
 
     PRUint32 SuspectedCount();
@@ -2499,8 +2499,12 @@ nsCycleCollector::Freed(void *n)
 #endif
 
 PRBool
-nsCycleCollector::PrepareForCollection(nsTPtrArray<PtrInfo> *aWhiteNodes)
+nsCycleCollector::PrepareForCollection(nsTPtrArray<PtrInfo> *aWhiteNodes,
+                                       PRBool aForceGC)
 {
+    NS_ASSERTION(NS_IsMainThread(),
+                 "PrepareForCollection must be called on the main thread.");
+
 #if defined(DEBUG_CC) && !defined(__MINGW32__)
     if (!mParams.mDoNothing && mParams.mHookMalloc)
         InitMemHook();
@@ -2529,6 +2533,29 @@ nsCycleCollector::PrepareForCollection(nsTPtrArray<PtrInfo> *aWhiteNodes)
 
     mWhiteNodes = aWhiteNodes;
 
+    // The cycle collector uses the mark bitmap to discover what JS objects
+    // were reachable only from XPConnect roots that might participate in
+    // cycles. We ask the JS runtime whether we need to force a GC before
+    // this CC. It returns true on startup (before the mark bits have been set),
+    // and also when UnmarkGray has run out of stack.
+    if (mRuntimes[nsIProgrammingLanguage::JAVASCRIPT]) {
+        nsCycleCollectionJSRuntime* rt =
+            static_cast<nsCycleCollectionJSRuntime*>
+                (mRuntimes[nsIProgrammingLanguage::JAVASCRIPT]);
+        if (rt->NeedCollect() || aForceGC) {
+#ifdef COLLECT_TIME_DEBUG
+            PRTime start = PR_Now();
+#endif
+            // rt->Collect() must be called from the main thread,
+            // because it invokes XPCJSRuntime::GCCallback(cx, JSGC_BEGIN)
+            // which returns false if not in the main thread.
+            rt->Collect();
+#ifdef COLLECT_TIME_DEBUG
+            printf("cc: GC() took %lldms\n", (PR_Now() - start) / PR_USEC_PER_MSEC);
+#endif
+        }
+    }
+
     return PR_TRUE;
 }
 
@@ -2563,13 +2590,13 @@ nsCycleCollector::Collect(PRUint32 aTryCollections,
 {
     nsAutoTPtrArray<PtrInfo, 4000> whiteNodes;
 
-    if (!PrepareForCollection(&whiteNodes))
+    if (!PrepareForCollection(&whiteNodes, PR_TRUE))
         return 0;
 
     PRUint32 totalCollections = 0;
     while (aTryCollections > totalCollections) {
         // Synchronous cycle collection. Always force a JS GC as well.
-        if (!(BeginCollection(PR_TRUE, aListener) && FinishCollection()))
+        if (!(BeginCollection(aListener) && FinishCollection()))
             break;
 
         ++totalCollections;
@@ -2581,32 +2608,11 @@ nsCycleCollector::Collect(PRUint32 aTryCollections,
 }
 
 PRBool
-nsCycleCollector::BeginCollection(PRBool aForceGC,
-                                  nsICycleCollectorListener *aListener)
+nsCycleCollector::BeginCollection(nsICycleCollectorListener *aListener)
 {
     if (mParams.mDoNothing)
         return PR_FALSE;
 
-    // The cycle collector uses the mark bitmap to discover what JS objects
-    // were reachable only from XPConnect roots that might participate in
-    // cycles. We ask the JS runtime whether we need to force a GC before
-    // this CC. It returns true on startup (before the mark bits have been set),
-    // and also when UnmarkGray has run out of stack.
-    if (mRuntimes[nsIProgrammingLanguage::JAVASCRIPT]) {
-        nsCycleCollectionJSRuntime* rt =
-            static_cast<nsCycleCollectionJSRuntime*>
-                (mRuntimes[nsIProgrammingLanguage::JAVASCRIPT]);
-        if (rt->NeedCollect() || aForceGC) {
-#ifdef COLLECT_TIME_DEBUG
-            PRTime start = PR_Now();
-#endif
-            rt->Collect();
-#ifdef COLLECT_TIME_DEBUG
-            printf("cc: GC() took %lldms\n", (PR_Now() - start) / PR_USEC_PER_MSEC);
-#endif
-        }
-    }
-
     if (aListener && NS_FAILED(aListener->Begin())) {
         aListener = nsnull;
     }
@@ -3358,7 +3364,7 @@ public:
                 return NS_OK;
             }
 
-            mCollected = mCollector->BeginCollection(PR_FALSE, mListener);
+            mCollected = mCollector->BeginCollection(mListener);
 
             mReply.Notify();
         }
@@ -3389,7 +3395,7 @@ public:
             return 0;
 
         nsAutoTPtrArray<PtrInfo, 4000> whiteNodes;
-        if (!mCollector->PrepareForCollection(&whiteNodes))
+        if (!mCollector->PrepareForCollection(&whiteNodes, PR_FALSE))
             return 0;
 
         NS_ASSERTION(!mListener, "Should have cleared this already!");