split the ghost task into implicit ghost_in and ghost_out tasks.

src/cell.c

@@ -1752,7 +1752,8 @@ int cell_unskip_tasks(struct cell *c, struct scheduler *s) {
   for (struct link *l = c->grav; l != NULL; l = l->next)
     scheduler_activate(s, l->t);
   if (c->extra_ghost != NULL) scheduler_activate(s, c->extra_ghost);
-  if (c->ghost != NULL) scheduler_activate(s, c->ghost);
+  if (c->ghost_in != NULL) scheduler_activate(s, c->ghost_in);
+  if (c->ghost_out != NULL) scheduler_activate(s, c->ghost_out);
   if (c->init_grav != NULL) scheduler_activate(s, c->init_grav);
   if (c->drift != NULL) scheduler_activate(s, c->drift);
   if (c->kick1 != NULL) scheduler_activate(s, c->kick1);

src/cell.h

@@ -151,8 +151,9 @@ struct cell {
   /*! The multipole initialistation task */
   struct task *init_grav;
 
-  /*! The ghost task */
-  struct task *ghost;
+  /*! The ghost tasks */
+  struct task *ghost_in;
+  struct task *ghost_out;
 
   /*! The extra ghost task for complex hydro schemes */
   struct task *extra_ghost;

src/engine.c

@@ -183,10 +183,13 @@ void engine_make_hierarchical_tasks(struct engine *e, struct cell *c) {
         scheduler_addunlock(s, c->grav_down, c->kick2);
       }
 
-      /* Generate the ghost task. */
-      if (is_hydro)
-        c->ghost = scheduler_addtask(s, task_type_ghost, task_subtype_none, 0,
-                                     0, c, NULL);
+      /* Generate the ghost tasks. */
+      if (is_hydro) {
+        c->ghost_in = scheduler_addtask(s, task_type_ghost, task_subtype_none, 0,
+                                     /* implicit = */ 1, c, NULL);
+        c->ghost_out = scheduler_addtask(s, task_type_ghost, task_subtype_none, 0,
+                                     /* implicit = */ 1, c, NULL);
+      }
 
 #ifdef EXTRA_HYDRO_LOOP
       /* Generate the extra ghost task. */
@@ -1041,10 +1044,10 @@ void engine_addtasks_send(struct engine *e, struct cell *ci, struct cell *cj,
       scheduler_addunlock(s, t_rho, ci->super->kick2);
 
       /* The send_rho task depends on the cell's ghost task. */
-      scheduler_addunlock(s, ci->super->ghost, t_rho);
+      scheduler_addunlock(s, ci->super->ghost_out, t_rho);
 
       /* The send_xv task should unlock the super-cell's ghost task. */
-      scheduler_addunlock(s, t_xv, ci->super->ghost);
+      scheduler_addunlock(s, t_xv, ci->super->ghost_in);
 
 #endif
 
@@ -2035,8 +2038,8 @@ static inline void engine_make_hydro_loops_dependencies(struct scheduler *sched,
                                                         struct cell *c,
                                                         int with_cooling) {
   /* density loop --> ghost --> force loop */
-  scheduler_addunlock(sched, density, c->super->ghost);
-  scheduler_addunlock(sched, c->super->ghost, force);
+  scheduler_addunlock(sched, density, c->super->ghost_in);
+  scheduler_addunlock(sched, c->super->ghost_out, force);
 
   if (with_cooling) {
     /* force loop --> cooling (--> kick2)  */

src/space.c

@@ -213,7 +213,8 @@ void space_rebuild_recycle_mapper(void *map_data, int num_elements,
     c->scount = 0;
     c->init_grav = NULL;
     c->extra_ghost = NULL;
-    c->ghost = NULL;
+    c->ghost_in = NULL;
+    c->ghost_out = NULL;
     c->kick1 = NULL;
     c->kick2 = NULL;
     c->timestep = NULL;