Threads don't need to always go to 'grayagain'

In incremental mode, threads don't need to be visited again once
visited in the atomic phase. In generational mode (where all visits
are in the atomic phase), only old threads need to be kept in the
'grayagain' list for the next cycle.

lgc.c

@@ -348,8 +348,7 @@ static int remarkupvals (global_State *g) {
   int work = 0;  /* estimate of how much work was done here */
   while ((thread = *p) != NULL) {
     work++;
-    lua_assert(!isblack(thread));  /* threads are never black */
-    if (isgray(thread) && thread->openupval != NULL)
+    if (!iswhite(thread) && thread->openupval != NULL)
       p = &thread->twups;  /* keep marked thread with upvalues in the list */
     else {  /* thread is not marked or without upvalues */
       UpVal *uv;
@@ -600,12 +599,23 @@ static int traverseLclosure (global_State *g, LClosure *cl) {
 
 /*
 ** Traverse a thread, marking the elements in the stack up to its top
-** and cleaning the rest of the stack in the final traversal.
-** That ensures that the entire stack have valid (non-dead) objects.
+** and cleaning the rest of the stack in the final traversal. That
+** ensures that the entire stack have valid (non-dead) objects.
+** Threads have no barriers. In gen. mode, old threads must be visited
+** at every cycle, because they might point to young objects.  In inc.
+** mode, the thread can still be modified before the end of the cycle,
+** and therefore it must be visited again in the atomic phase. To ensure
+** these visits, threads must return to a gray list if they are not new
+** (which can only happen in generational mode) or if the traverse is in
+** the propagate phase (which can only happen in incremental mode).
 */
 static int traversethread (global_State *g, lua_State *th) {
   UpVal *uv;
   StkId o = th->stack;
+  if (isold(th) || g->gcstate == GCSpropagate) {
+    linkgclist(th, g->grayagain);  /* insert into 'grayagain' list */
+    black2gray(th);
+  }
   if (o == NULL)
     return 1;  /* stack not completely built yet */
   lua_assert(g->gcstate == GCSatomic ||
@@ -644,12 +654,7 @@ static lu_mem propagatemark (global_State *g) {
     case LUA_VLCL: return traverseLclosure(g, gco2lcl(o));
     case LUA_VCCL: return traverseCclosure(g, gco2ccl(o));
     case LUA_VPROTO: return traverseproto(g, gco2p(o));
-    case LUA_VTHREAD: {
-      lua_State *th = gco2th(o);
-      linkgclist(th, g->grayagain);  /* insert into 'grayagain' list */
-      black2gray(o);
-      return traversethread(g, th);
-    }
+    case LUA_VTHREAD: return traversethread(g, gco2th(o));
     default: lua_assert(0); return 0;
   }
 }
@@ -1028,7 +1033,6 @@ static void setpause (global_State *g);
 ** objects and turns the non dead to old. All non-dead threads---which
 ** are now old---must be in a gray list.  Everything else is not in a
 ** gray list.
-**
 */
 static void sweep2old (lua_State *L, GCObject **p) {
   GCObject *curr;
@@ -1139,10 +1143,16 @@ static GCObject **correctgraylist (GCObject **p) {
       goto remain;  /* keep non-white threads on the list */
     }
     else {  /* everything else is removed */
-      lua_assert(isold(curr));  /* young objects should be white */
-      if (getage(curr) == G_TOUCHED2)  /* advance from G_TOUCHED2... */
-        changeage(curr, G_TOUCHED2, G_OLD);  /* ... to G_OLD */
-      gray2black(curr);  /* make object black */
+      lua_assert(isold(curr));  /* young objects should be white here */
+      if (getage(curr) == G_TOUCHED2) {  /* advance from TOUCHED2... */
+        changeage(curr, G_TOUCHED2, G_OLD);  /* ... to OLD */
+        lua_assert(isblack(curr));  /* TOUCHED2 objects are always black */
+      }
+      else {
+        /* everything else in a gray list should be gray */
+        lua_assert(isgray(curr));
+        gray2black(curr);  /* make object black (to be removed) */
+      }
       goto remove;
     }
     remove: *p = *next; continue;
@@ -1207,11 +1217,12 @@ static void youngcollection (lua_State *L, global_State *g) {
   GCObject **psurvival;  /* to point to first non-dead survival object */
   GCObject *dummy;  /* dummy out parameter to 'sweepgen' */
   lua_assert(g->gcstate == GCSpropagate);
-  if (g->firstold1) {  /* are there OLD1 objects? */
+  if (g->firstold1) {  /* are there regular OLD1 objects? */
     markold(g, g->firstold1, g->reallyold);  /* mark them */
     g->firstold1 = NULL;  /* no more OLD1 objects (for now) */
   }
   markold(g, g->finobj, g->finobjrold);
+  markold(g, g->tobefnz, NULL);
   atomic(L);
 
   /* sweep nursery and get a pointer to its last live element */
@@ -1268,8 +1279,8 @@ static void atomic2gen (lua_State *L, global_State *g) {
 /*
 ** Enter generational mode. Must go until the end of an atomic cycle
 ** to ensure that all objects are correctly marked and weak tables
-** are cleared.
-** Then, turn all objects into old and finishes the collection.
+** are cleared. Then, turn all objects into old and finishes the
+** collection.
 */
 static lu_mem entergen (lua_State *L, global_State *g) {
   lu_mem numobjs;