Implementation of true asynchronous signaling between processes

Communication between Erlang processes has conceptually always been
performed through asynchronous signaling. The runtime system
implementation has however previously preformed most operation
synchronously. In a system with only one true thread of execution, this
is not problematic (often the opposite). In a system with multiple threads
of execution (as current runtime system implementation with SMP support)
it becomes problematic. This since it often involves locking of structures
when updating them which in turn cause resource contention. Utilizing
true asynchronous communication often avoids these resource contention
issues.

The case that triggered this change was contention on the link lock due
to frequent updates of the monitor trees during communication with a
frequently used server. The signal order delivery guarantees of the
language makes it hard to change the implementation of only some signals
to use true asynchronous signaling. Therefore the implementations
of (almost) all signals have been changed.

Currently the following signals have been implemented as true
asynchronous signals:
- Message signals
- Exit signals
- Monitor signals
- Demonitor signals
- Monitor triggered signals (DOWN, CHANGE, etc)
- Link signals
- Unlink signals
- Group leader signals

All of the above already defined as asynchronous signals in the
language. The implementation of messages signals was quite
asynchronous to begin with, but had quite strict delivery constraints
due to the ordering guarantees of signals between a pair of processes.

The previously used message queue partitioned into two halves has been
replaced by a more general signal queue partitioned into three parts
that service all kinds of signals. More details regarding the signal
queue can be found in comments in the erl_proc_sig_queue.h file.

The monitor and link implementations have also been completely replaced
in order to fit the new asynchronous signaling implementation as good
as possible. More details regarding the new monitor and link
implementations can be found in the erl_monitor_link.h file.

1 files changed, 307 insertions, 101 deletions

diff --git a/erts/emulator/beam/erl_node_tables.c b/erts/emulator/beam/erl_node_tables.c
index e8901a652f..ca83e70046 100644
--- a/erts/emulator/beam/erl_node_tables.c
+++ b/erts/emulator/beam/erl_node_tables.c
@@ -1,7 +1,7 @@
 /*
  * %CopyrightBegin%
  *
- * Copyright Ericsson AB 2001-2017. All Rights Reserved.
+ * Copyright Ericsson AB 2001-2018. All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@
 #include "dtrace-wrapper.h"
 #include "erl_binary.h"
 #include "erl_bif_unique.h"
+#include "erl_proc_sig_queue.h"
 
 Hash erts_dist_table;
 Hash erts_node_table;
@@ -174,11 +175,7 @@ dist_table_alloc(void *dep_tmpl)
     dep->flags				= 0;
     dep->version			= 0;
 
-    erts_mtx_init(&dep->lnk_mtx, "dist_entry_links", sysname,
-        ERTS_LOCK_FLAGS_CATEGORY_DISTRIBUTION);
-    dep->node_links			= NULL;
-    dep->nlinks				= NULL;
-    dep->monitors			= NULL;
+    dep->mld                            = NULL;
 
     erts_mtx_init(&dep->qlock, "dist_entry_out_queue", sysname,
         ERTS_LOCK_FLAGS_CATEGORY_DISTRIBUTION);
@@ -225,9 +222,7 @@ dist_table_free(void *vdep)
     ASSERT(de_refc_read(dep, -1) == -1);
     ASSERT(dep->state == ERTS_DE_STATE_IDLE);
     ASSERT(is_nil(dep->cid));
-    ASSERT(dep->nlinks == NULL);
-    ASSERT(dep->node_links == NULL);
-    ASSERT(dep->monitors == NULL);
+    ASSERT(dep->mld == NULL);
 
     /* Link out */
 
@@ -250,7 +245,6 @@ dist_table_free(void *vdep)
 
     ASSERT(!dep->cache);
     erts_rwmtx_destroy(&dep->rwmtx);
-    erts_mtx_destroy(&dep->lnk_mtx);
     erts_mtx_destroy(&dep->qlock);
 
 #ifdef DEBUG
@@ -606,7 +600,9 @@ erts_set_dist_entry_not_connected(DistEntry *dep)
 void
 erts_set_dist_entry_pending(DistEntry *dep)
 {
+    ErtsMonLnkDist *mld = erts_mon_link_dist_create(dep->sysname);
     ERTS_LC_ASSERT(erts_lc_is_de_rwlocked(dep));
+    
     erts_rwmtx_rwlock(&erts_dist_table_rwmtx);
 
     ASSERT(dep != erts_this_dist_entry);
@@ -631,6 +627,10 @@ erts_set_dist_entry_pending(DistEntry *dep)
     dep->flags = (DFLAG_DIST_MANDATORY | DFLAG_DIST_HOPEFULLY | DFLAG_NO_MAGIC);
     dep->connection_id = (dep->connection_id + 1) & ERTS_DIST_CON_ID_MASK;
 
+    ASSERT(!dep->mld);
+    mld->connection_id = dep->connection_id;
+    dep->mld = mld;
+
     dep->prev = NULL;
     dep->next = erts_pending_dist_entries;
     if(erts_pending_dist_entries) {
@@ -647,6 +647,8 @@ erts_set_dist_entry_connected(DistEntry *dep, Eterm cid, Uint flags)
 {
     erts_aint32_t set_qflgs;
 
+    ASSERT(dep->mld);
+
     ERTS_LC_ASSERT(erts_lc_is_de_rwlocked(dep));
     erts_rwmtx_rwlock(&erts_dist_table_rwmtx);
 
@@ -1099,6 +1101,7 @@ static Eterm AM_system;
 static Eterm AM_timer;
 static Eterm AM_delayed_delete_timer;
 static Eterm AM_thread_progress_delete_timer;
+static Eterm AM_signal;
 
 static void setup_reference_table(void);
 static Eterm reference_table_term(Uint **hpp, ErlOffHeap *ohp, Uint *szp);
@@ -1120,6 +1123,7 @@ typedef struct node_referrer_ {
     int bin_ref;
     int timer_ref;
     int system_ref;
+    int signal_ref;
     Eterm id;
     Uint id_heap[ID_HEAP_SIZE];
     ErlOffHeap off_heap;
@@ -1137,6 +1141,7 @@ typedef struct dist_referrer_ {
     int node_ref;
     int ctrl_ref;
     int system_ref;
+    int signal_ref;
     Eterm id;
     Uint creation;
     Uint id_heap[ID_HEAP_SIZE];
@@ -1190,6 +1195,7 @@ erts_get_node_and_dist_references(struct process *proc)
 	INIT_AM(system);
 	INIT_AM(delayed_delete_timer);
         INIT_AM(thread_progress_delete_timer);
+	INIT_AM(signal);
 	references_atoms_need_init = 0;
     }
 
@@ -1226,6 +1232,7 @@ erts_get_node_and_dist_references(struct process *proc)
 #define MONITOR_REF 7
 #define TIMER_REF 8
 #define SYSTEM_REF 9
+#define SIGNAL_REF 10
 
 #define INC_TAB_SZ 10
 
@@ -1260,6 +1267,7 @@ insert_dist_referrer(ReferredDist *referred_dist,
 	drp->node_ref = 0;
 	drp->ctrl_ref = 0;
 	drp->system_ref = 0;
+        drp->signal_ref = 0;
     }
 
     switch (type) {
@@ -1268,6 +1276,7 @@ insert_dist_referrer(ReferredDist *referred_dist,
     case HEAP_REF:	drp->heap_ref++;	break;
     case ETS_REF:	drp->ets_ref++;	        break;
     case SYSTEM_REF:	drp->system_ref++;	break;
+    case SIGNAL_REF:	drp->signal_ref++;	break;
     default:		ASSERT(0);
     }
 }
@@ -1321,6 +1330,7 @@ insert_node_referrer(ReferredNode *referred_node, int type, Eterm id)
 	nrp->bin_ref = 0;
 	nrp->timer_ref = 0;
 	nrp->system_ref = 0;
+	nrp->signal_ref = 0;
     }
 
     switch (type) {
@@ -1331,6 +1341,7 @@ insert_node_referrer(ReferredNode *referred_node, int type, Eterm id)
     case MONITOR_REF:   nrp->monitor_ref++;     break;
     case TIMER_REF:	nrp->timer_ref++;	break;
     case SYSTEM_REF:	nrp->system_ref++;	break;
+    case SIGNAL_REF:	nrp->signal_ref++;	break;
     default:		ASSERT(0);
     }
 }
@@ -1438,49 +1449,145 @@ insert_offheap(ErlOffHeap *oh, int type, Eterm id)
     }
 }
 
-static void doit_insert_monitor(ErtsMonitor *monitor, void *p)
+static void insert_monitor_data(ErtsMonitor *mon, int type, Eterm id)
+{
+    ErtsMonitorData *mdp = erts_monitor_to_data(mon);
+    if ((mdp->origin.flags & (ERTS_ML_FLG_DBG_VISITED
+                              | ERTS_ML_FLG_EXTENDED)) == ERTS_ML_FLG_EXTENDED) {
+        if (mon->type != ERTS_MON_TYPE_NODE) {
+            ErtsMonitorDataExtended *mdep = (ErtsMonitorDataExtended *) mdp;
+            ASSERT(mon->flags & ERTS_ML_FLG_EXTENDED);
+            if (mdep->uptr.ohhp) {
+                ErlOffHeap oh;
+                ERTS_INIT_OFF_HEAP(&oh);
+                oh.first = mdep->uptr.ohhp;
+                insert_offheap(&oh, type, id);
+            }
+        }
+    }
+    mdp->origin.flags |= ERTS_ML_FLG_DBG_VISITED;
+}
+
+static void insert_monitor(ErtsMonitor *mon, void *idp)
+{
+    Eterm id = *((Eterm *) idp);
+    insert_monitor_data(mon, MONITOR_REF, id);
+}
+
+static void clear_visited_monitor(ErtsMonitor *mon, void *p)
+{
+    ErtsMonitorData *mdp = erts_monitor_to_data(mon);
+    mdp->origin.flags &= ~ERTS_ML_FLG_DBG_VISITED;
+}
+
+static void
+insert_p_monitors(ErtsPTabElementCommon *p)
+{
+    Eterm id = p->id;
+    erts_monitor_tree_foreach(p->u.alive.monitors,
+                              insert_monitor,
+                              (void *) &id);
+    erts_monitor_list_foreach(p->u.alive.lt_monitors,
+                              insert_monitor,
+                              (void *) &id);
+}
+
+static void
+insert_dist_monitors(DistEntry *dep)
+{
+    if (dep->mld) {
+        erts_monitor_list_foreach(dep->mld->monitors,
+                                  insert_monitor,
+                                  (void *) &dep->sysname);
+        erts_monitor_tree_foreach(dep->mld->orig_name_monitors,
+                                  insert_monitor,
+                                  (void *) &dep->sysname);
+    }
+}
+
+static void
+clear_visited_p_monitors(ErtsPTabElementCommon *p)
+{
+    erts_monitor_tree_foreach(p->u.alive.monitors,
+                              clear_visited_monitor,
+                              NULL);
+    erts_monitor_list_foreach(p->u.alive.lt_monitors,
+                              clear_visited_monitor,
+                              NULL);
+}
+
+static void
+clear_visited_dist_monitors(DistEntry *dep)
+{
+    if (dep->mld) {
+        erts_monitor_list_foreach(dep->mld->monitors,
+                                  clear_visited_monitor,
+                                  NULL);
+        erts_monitor_tree_foreach(dep->mld->orig_name_monitors,
+                                  clear_visited_monitor,
+                                  NULL);
+    }
+}
+
+static void insert_link_data(ErtsLink *lnk, int type, Eterm id)
 {
-    Eterm *idp = p;
-    if(monitor->type != MON_NIF_TARGET && is_external(monitor->u.pid))
-	insert_node(external_thing_ptr(monitor->u.pid)->node, MONITOR_REF, *idp);
-    if(is_external(monitor->ref))
-	insert_node(external_thing_ptr(monitor->ref)->node, MONITOR_REF, *idp);
+    ErtsLinkData *ldp = erts_link_to_data(lnk);
+    if ((ldp->a.flags & (ERTS_ML_FLG_DBG_VISITED
+                         | ERTS_ML_FLG_EXTENDED)) == ERTS_ML_FLG_EXTENDED) {
+        ErtsLinkDataExtended *ldep = (ErtsLinkDataExtended *) ldp;
+        if (ldep->ohhp) {
+            ErlOffHeap oh;
+            ERTS_INIT_OFF_HEAP(&oh);
+            oh.first = ldep->ohhp;
+            insert_offheap(&oh, type, id);
+        }
+    }
+    ldp->a.flags |= ERTS_ML_FLG_DBG_VISITED;
 }
 
-static void doit_insert_link(ErtsLink *lnk, void *p)
+static void insert_link(ErtsLink *lnk, void *idp)
 {
-    Eterm *idp = p;
-    if(is_external(lnk->pid))
-	insert_node(external_thing_ptr(lnk->pid)->node, LINK_REF, 
-		    *idp);
+    Eterm id = *((Eterm *) idp);
+    insert_link_data(lnk, LINK_REF, id);
 }
 
+static void clear_visited_link(ErtsLink *lnk, void *p)
+{
+    ErtsLinkData *ldp = erts_link_to_data(lnk);
+    ldp->a.flags &= ~ERTS_ML_FLG_DBG_VISITED;
+}
 
 static void
-insert_monitors(ErtsMonitor *monitors, Eterm id)
+insert_p_links(ErtsPTabElementCommon *p)
 {
-    erts_doforall_monitors(monitors,&doit_insert_monitor,&id);
+    Eterm id = p->id;
+    erts_link_tree_foreach(p->u.alive.links, insert_link, (void *) &id);
 }
 
 static void
-insert_links(ErtsLink *lnk, Eterm id)
+insert_dist_links(DistEntry *dep)
 {
-    erts_doforall_links(lnk,&doit_insert_link,&id);
+    if (dep->mld)
+        erts_link_list_foreach(dep->mld->links,
+                               insert_link,
+                               (void *) &dep->sysname);
 }
 
-static void doit_insert_link2(ErtsLink *lnk, void *p)
+static void
+clear_visited_p_links(ErtsPTabElementCommon *p)
 {
-    Eterm *idp = p;
-    if(is_external(lnk->pid))
-	insert_node(external_thing_ptr(lnk->pid)->node, LINK_REF, 
-		    *idp);
-    insert_links(ERTS_LINK_ROOT(lnk), *idp);
+    erts_link_tree_foreach(p->u.alive.links,
+                           clear_visited_link,
+                           NULL);
 }
 
 static void
-insert_links2(ErtsLink *lnk, Eterm id)
+clear_visited_dist_links(DistEntry *dep)
 {
-    erts_doforall_links(lnk,&doit_insert_link2,&id);
+    if (dep->mld)
+        erts_link_list_foreach(dep->mld->links,
+                               clear_visited_link,
+                               NULL);
 }
 
 static void
@@ -1576,6 +1683,60 @@ insert_delayed_delete_dist_entry(void *state,
 }
 
 static void
+insert_message(ErtsMessage *msg, int type, Process *proc)
+{
+    ErlHeapFragment *heap_frag = NULL;
+
+    ASSERT(ERTS_SIG_IS_MSG(msg));
+    if (msg->data.attached) {
+        if (msg->data.attached == ERTS_MSG_COMBINED_HFRAG)
+            heap_frag = &msg->hfrag;
+        else if (ERTS_SIG_IS_INTERNAL_MSG(msg))
+            heap_frag = msg->data.heap_frag;
+        else {
+            if (msg->data.dist_ext->dep)
+                insert_dist_entry(msg->data.dist_ext->dep,
+                                  type, proc->common.id, 0);
+            if (is_not_nil(ERL_MESSAGE_TOKEN(msg)))
+                heap_frag = erts_dist_ext_trailer(msg->data.dist_ext);
+        }
+    }
+    while (heap_frag) {
+        insert_offheap(&(heap_frag->off_heap),
+                       type,
+                       proc->common.id);
+        heap_frag = heap_frag->next;
+    }
+}
+
+static void
+insert_sig_msg(ErtsMessage *msg, void *arg)
+{
+    insert_message(msg, SIGNAL_REF, (Process *) arg);
+}
+
+static void
+insert_sig_offheap(ErlOffHeap *ohp, void *arg)
+{
+    Process *proc = arg;
+    insert_offheap(ohp, SIGNAL_REF, proc->common.id);
+}
+
+static void
+insert_sig_monitor(ErtsMonitor *mon, void *arg)
+{
+    Process *proc = arg;
+    insert_monitor_data(mon, SIGNAL_REF, proc->common.id);
+}
+
+static void
+insert_sig_link(ErtsLink *lnk, void *arg)
+{
+    Process *proc = arg;
+    insert_link_data(lnk, SIGNAL_REF, proc->common.id);
+}
+
+static void
 setup_reference_table(void)
 {
     ErlHeapFragment *hfp;
@@ -1630,10 +1791,7 @@ setup_reference_table(void)
 	Process *proc = erts_pix2proc(i);
 	if (proc) {
 	    int mli;
-	    ErtsMessage *msg_list[] = {
-		proc->msg.first,
-		proc->msg_inq.first,
-		proc->msg_frag};
+	    ErtsMessage *msg_list[] = {proc->msg_frag};
 
 	    /* Insert Heap */
 	    insert_offheap(&(proc->off_heap),
@@ -1648,34 +1806,24 @@ setup_reference_table(void)
 	    /* Insert msg buffers */
 	    for (mli = 0; mli < sizeof(msg_list)/sizeof(msg_list[0]); mli++) {
 		ErtsMessage *msg;
-		for (msg = msg_list[mli]; msg; msg = msg->next) {
-		    ErlHeapFragment *heap_frag = NULL;
-		    if (msg->data.attached) {
-			if (msg->data.attached == ERTS_MSG_COMBINED_HFRAG)
-			    heap_frag = &msg->hfrag;
-			else if (is_value(ERL_MESSAGE_TERM(msg)))
-			    heap_frag = msg->data.heap_frag;
-			else {
-			    if (msg->data.dist_ext->dep)
-				insert_dist_entry(msg->data.dist_ext->dep,
-						  HEAP_REF, proc->common.id, 0);
-			    if (is_not_nil(ERL_MESSAGE_TOKEN(msg)))
-				heap_frag = erts_dist_ext_trailer(msg->data.dist_ext);
-			}
-		    }
-		    while (heap_frag) {
-			insert_offheap(&(heap_frag->off_heap),
-				       HEAP_REF,
-				       proc->common.id);
-			heap_frag = heap_frag->next;
-		    }
-		}
+		for (msg = msg_list[mli]; msg; msg = msg->next)
+                    insert_message(msg, HEAP_REF, proc);
 	    }
+
+            /* Insert signal queue */
+            erts_proc_sig_debug_foreach_sig(proc,
+                                            insert_sig_msg,
+                                            insert_sig_offheap,
+                                            insert_sig_monitor,
+                                            insert_sig_link,
+                                            (void *) proc);
+
 	    /* Insert links */
-	    if (ERTS_P_LINKS(proc))
-		insert_links(ERTS_P_LINKS(proc), proc->common.id);
-	    if (ERTS_P_MONITORS(proc))
-		insert_monitors(ERTS_P_MONITORS(proc), proc->common.id);
+            insert_p_links(&proc->common);
+
+            /* Insert monitors */
+            insert_p_monitors(&proc->common);
+
             {
                 DistEntry *dep = ERTS_PROC_GET_DIST_ENTRY(proc);
                 if (dep)
@@ -1705,11 +1853,9 @@ setup_reference_table(void)
 	    continue;
 
 	/* Insert links */
-	if (ERTS_P_LINKS(prt))
-	    insert_links(ERTS_P_LINKS(prt), prt->common.id);
+        insert_p_links(&prt->common);
 	/* Insert monitors */
-	if (ERTS_P_MONITORS(prt))
-	    insert_monitors(ERTS_P_MONITORS(prt), prt->common.id);
+        insert_p_monitors(&prt->common);
 	/* Insert port data */
 	ohp = erts_port_data_offheap(prt);
 	if (ohp)
@@ -1758,41 +1904,25 @@ setup_reference_table(void)
     /* Insert all dist links */
 
     for(dep = erts_visible_dist_entries; dep; dep = dep->next) {
-	if(dep->nlinks)
-	    insert_links2(dep->nlinks, dep->sysname);
-	if(dep->node_links)
-	    insert_links(dep->node_links, dep->sysname);
-	if(dep->monitors)
-	    insert_monitors(dep->monitors, dep->sysname);
+        insert_dist_links(dep);
+        insert_dist_monitors(dep);
     }
 
     for(dep = erts_hidden_dist_entries; dep; dep = dep->next) {
-	if(dep->nlinks)
-	    insert_links2(dep->nlinks, dep->sysname);
-	if(dep->node_links)
-	    insert_links(dep->node_links, dep->sysname);
-	if(dep->monitors)
-	    insert_monitors(dep->monitors, dep->sysname);
+        insert_dist_links(dep);
+        insert_dist_monitors(dep);
     }
 
     for(dep = erts_pending_dist_entries; dep; dep = dep->next) {
-        if(dep->nlinks)
-            insert_links2(dep->nlinks, dep->sysname);
-        if(dep->node_links)
-            insert_links(dep->node_links, dep->sysname);
-        if(dep->monitors)
-            insert_monitors(dep->monitors, dep->sysname);
+        insert_dist_links(dep);
+        insert_dist_monitors(dep);
     }
 
     /* Not connected dist entries should not have any links,
        but inspect them anyway */
     for(dep = erts_not_connected_dist_entries; dep; dep = dep->next) {
-	if(dep->nlinks)
-	    insert_links2(dep->nlinks, dep->sysname);
-	if(dep->node_links)
-	    insert_links(dep->node_links, dep->sysname);
-	if(dep->monitors)
-	    insert_monitors(dep->monitors, dep->sysname);
+        insert_dist_links(dep);
+        insert_dist_monitors(dep);
     }
 
     /* Insert all ets tables */
@@ -1877,6 +2007,10 @@ reference_table_term(Uint **hpp, ErlOffHeap *ohp, Uint *szp)
 		tup = MK_2TUP(AM_system, MK_UINT(nrp->system_ref));
 		nrl = MK_CONS(tup, nrl);
 	    }
+	    if(nrp->signal_ref) {
+		tup = MK_2TUP(AM_signal, MK_UINT(nrp->signal_ref));
+		nrl = MK_CONS(tup, nrl);
+	    }
 
 	    nrid = nrp->id;
 	    if (!IS_CONST(nrp->id)) {
@@ -1900,24 +2034,25 @@ reference_table_term(Uint **hpp, ErlOffHeap *ohp, Uint *szp)
 	    }
 	    else if(is_internal_port(nrid)) {
 		ASSERT(!nrp->heap_ref && !nrp->ets_ref && !nrp->bin_ref
-		       && !nrp->timer_ref && !nrp->system_ref);
+		       && !nrp->timer_ref && !nrp->system_ref && !nrp->signal_ref);
 		tup = MK_2TUP(AM_port, nrid);
 	    }
 	    else if(nrp->ets_ref) {
 		ASSERT(!nrp->heap_ref && !nrp->link_ref && 
 		       !nrp->monitor_ref && !nrp->bin_ref
-		       && !nrp->timer_ref && !nrp->system_ref);
+		       && !nrp->timer_ref && !nrp->system_ref && !nrp->signal_ref);
 		tup = MK_2TUP(AM_ets, nrid);
 	    }
 	    else if(nrp->bin_ref) {
 		ASSERT(is_small(nrid) || is_big(nrid));
 		ASSERT(!nrp->heap_ref && !nrp->ets_ref && !nrp->link_ref && 
 		       !nrp->monitor_ref && !nrp->timer_ref
-		       && !nrp->system_ref);
+		       && !nrp->system_ref && !nrp->signal_ref);
 		tup = MK_2TUP(AM_match_spec, nrid);
 	    }
 	    else  {
-		ASSERT(!nrp->heap_ref && !nrp->ets_ref && !nrp->bin_ref);
+		ASSERT(!nrp->heap_ref && !nrp->ets_ref && !nrp->bin_ref
+                       && !nrp->signal_ref);
 		ASSERT(is_atom(nrid));
 		tup = MK_2TUP(AM_dist, nrid);
 	    }
@@ -1961,30 +2096,36 @@ reference_table_term(Uint **hpp, ErlOffHeap *ohp, Uint *szp)
 		tup = MK_2TUP(AM_system, MK_UINT(drp->system_ref));
 		drl = MK_CONS(tup, drl);
 	    }
+	    if(drp->signal_ref) {
+		tup = MK_2TUP(AM_signal, MK_UINT(drp->signal_ref));
+		drl = MK_CONS(tup, drl);
+	    }
 
 	    if (is_internal_pid(drp->id)) {
 		ASSERT(!drp->node_ref);
 		tup = MK_2TUP(AM_process, drp->id);
 	    }
 	    else if(is_internal_port(drp->id)) {
-		ASSERT(drp->ctrl_ref && !drp->node_ref);
+		ASSERT(drp->ctrl_ref && !drp->node_ref && !drp->signal_ref);
 		tup = MK_2TUP(AM_port, drp->id);
 	    }
 	    else if (is_tuple(drp->id)) {
 		Eterm *t;
 		ASSERT(drp->system_ref && !drp->node_ref
-		       && !drp->ctrl_ref && !drp->heap_ref && !drp->ets_ref);
+		       && !drp->ctrl_ref && !drp->heap_ref && !drp->ets_ref
+                       && !drp->signal_ref);
 		t = tuple_val(drp->id);
 		ASSERT(2 == arityval(t[0]));
 		tup = MK_2TUP(t[1], t[2]);
 	    }
 	    else if (drp->ets_ref) {
                 ASSERT(!drp->heap_ref && !drp->node_ref &&
-                       !drp->ctrl_ref && !drp->system_ref);
+                       !drp->ctrl_ref && !drp->system_ref
+                       && !drp->signal_ref);
                 tup = MK_2TUP(AM_ets, drp->id);
             }
-            else {
-		ASSERT(!drp->ctrl_ref && drp->node_ref);
+	    else {
+		ASSERT(!drp->ctrl_ref && drp->node_ref && !drp->signal_ref);
 		ASSERT(is_atom(drp->id));
 		tup = MK_2TUP(drp->id, MK_UINT(drp->creation));
 		tup = MK_2TUP(AM_node, tup);
@@ -2019,10 +2160,21 @@ reference_table_term(Uint **hpp, ErlOffHeap *ohp, Uint *szp)
 
 }
 
+static void noop_sig_msg(ErtsMessage *msg, void *arg)
+{
+
+}
+
+static void noop_sig_offheap(ErlOffHeap *oh, void *arg)
+{
+
+}
+
 static void
 delete_reference_table(void)
 {
-    Uint i;
+    DistEntry *dep;
+    int i, max;
     for(i = 0; i < no_referred_nodes; i++) {
 	NodeReferrer *nrp;
 	NodeReferrer *tnrp;
@@ -2054,6 +2206,60 @@ delete_reference_table(void)
 	inserted_bins = inserted_bins->next;
 	erts_free(ERTS_ALC_T_NC_TMP, (void *)ib);
     }
+
+    /* Cleanup... */
+
+    max = erts_ptab_max(&erts_proc);
+    for (i = 0; i < max; i++) {
+	Process *proc = erts_pix2proc(i);
+	if (proc) {
+            clear_visited_p_links(&proc->common);
+            clear_visited_p_monitors(&proc->common);
+            erts_proc_sig_debug_foreach_sig(proc,
+                                            noop_sig_msg,
+                                            noop_sig_offheap,
+                                            clear_visited_monitor,
+                                            clear_visited_link,
+                                            (void *) proc);
+        }
+    }
+
+    max = erts_ptab_max(&erts_port);
+    for (i = 0; i < max; i++) {
+	erts_aint32_t state;
+	Port *prt;
+
+	prt = erts_pix2port(i);
+	if (!prt)
+	    continue;
+
+	state = erts_atomic32_read_nob(&prt->state);
+	if (state & ERTS_PORT_SFLGS_DEAD)
+	    continue;
+
+        clear_visited_p_links(&prt->common);
+        clear_visited_p_monitors(&prt->common);
+    }
+
+    for(dep = erts_visible_dist_entries; dep; dep = dep->next) {
+        clear_visited_dist_links(dep);
+        clear_visited_dist_monitors(dep);
+    }
+
+    for(dep = erts_hidden_dist_entries; dep; dep = dep->next) {
+        clear_visited_dist_links(dep);
+        clear_visited_dist_monitors(dep);
+    }
+
+    for(dep = erts_pending_dist_entries; dep; dep = dep->next) {
+        clear_visited_dist_links(dep);
+        clear_visited_dist_monitors(dep);
+    }
+
+    for(dep = erts_not_connected_dist_entries; dep; dep = dep->next) {
+        clear_visited_dist_links(dep);
+        clear_visited_dist_monitors(dep);
+    }
 }
 
 void