Why GCTM stop gc step before calling userdata's finalizer?

[张伟智 <robotk@...>]

Lua version is 5.2.4.
In lgc.c line 820, function GCTM,
g->gcrunning is set to 0 to avoid GC steps,
I think if g->gcrunning remains unchanged,
line 824 luaD_pcall will trigger luaC_step->luaC_forcestep->GCTM during executing __gc finalizer.
But what is the problem if recursive enter GCTM ?

809 static void GCTM (lua_State *L, int propagateerrors) {
810   global_State *g = G(L);
811   const TValue *tm;
812   TValue v;
813   setgcovalue(L, &v, udata2finalize(g));
814   tm = luaT_gettmbyobj(L, &v, TM_GC);
815   if (tm != NULL && ttisfunction(tm)) {  /* is there a finalizer? */
816     int status;
817     lu_byte oldah = L->allowhook;
818     int running  = g->gcrunning;
819     L->allowhook = 0;  /* stop debug hooks during GC metamethod */
820     g->gcrunning = 0;  /* avoid GC steps */
821     setobj2s(L, L->top, tm);  /* push finalizer... */
822     setobj2s(L, L->top + 1, &v);  /* ... and its argument */
823     L->top += 2;  /* and (next line) call the finalizer */
824     status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
825     L->allowhook = oldah;  /* restore hooks */
826     g->gcrunning = running;  /* restore state */

1163 void luaC_forcestep (lua_State *L) {
1164   global_State *g = G(L);
1165   int i;
1166   if (isgenerational(g)) generationalcollection(L);
1167   else incstep(L);
1168   /* run a few finalizers (or all of them at the end of a collect cycle) */
1169   for (i = 0; g->tobefnz && (i < GCFINALIZENUM || g->gcstate == GCSpause); i++)
1170     GCTM(L, 1);  /* call one finalizer */
1171 }
1172 
1173 
1174 /*
1175 ** performs a basic GC step only if collector is running
1176 */
1177 void luaC_step (lua_State *L) {
1178   global_State *g = G(L);
1179   if (g->gcrunning) luaC_forcestep(L);
1180   else luaE_setdebt(g, -GCSTEPSIZE);  /* avoid being called too often */
1181 }