cgroup: factor out css_set_move_task()

A task is associated and disassociated with its css_set in three
places - during migration, after a new task is created and when a task
exits.  The first is handled by cgroup_task_migrate() and the latter
two are open-coded.

These are similar operations and spreading them over multiple places
makes it harder to follow and update.  This patch collects all task
css_set [dis]association operations into css_set_move_task().

While css_set_move_task() may check whether populated state needs to
be updated when not strictly necessary, the behavior is essentially
equivalent before and after this patch.

Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 52444673e9a2bc32aa9cfdab57a679c22268cd35..0b6e50c67b65ce2f4efa148e3db08a4f0d6c3ef8 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -664,6 +664,52 @@ static void css_set_update_populated(struct css_set *cset, bool populated)
                cgroup_update_populated(link->cgrp, populated);
 }
 
+/**
+ * css_set_move_task - move a task from one css_set to another
+ * @task: task being moved
+ * @from_cset: css_set @task currently belongs to (may be NULL)
+ * @to_cset: new css_set @task is being moved to (may be NULL)
+ * @use_mg_tasks: move to @to_cset->mg_tasks instead of ->tasks
+ *
+ * Move @task from @from_cset to @to_cset.  If @task didn't belong to any
+ * css_set, @from_cset can be NULL.  If @task is being disassociated
+ * instead of moved, @to_cset can be NULL.
+ *
+ * This function automatically handles populated_cnt updates but the caller
+ * is responsible for managing @from_cset and @to_cset's reference counts.
+ */
+static void css_set_move_task(struct task_struct *task,
+                             struct css_set *from_cset, struct css_set *to_cset,
+                             bool use_mg_tasks)
+{
+       lockdep_assert_held(&css_set_rwsem);
+
+       if (from_cset) {
+               WARN_ON_ONCE(list_empty(&task->cg_list));
+               list_del_init(&task->cg_list);
+               if (!css_set_populated(from_cset))
+                       css_set_update_populated(from_cset, false);
+       } else {
+               WARN_ON_ONCE(!list_empty(&task->cg_list));
+       }
+
+       if (to_cset) {
+               /*
+                * We are synchronized through cgroup_threadgroup_rwsem
+                * against PF_EXITING setting such that we can't race
+                * against cgroup_exit() changing the css_set to
+                * init_css_set and dropping the old one.
+                */
+               WARN_ON_ONCE(task->flags & PF_EXITING);
+
+               if (!css_set_populated(to_cset))
+                       css_set_update_populated(to_cset, true);
+               rcu_assign_pointer(task->cgroups, to_cset);
+               list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
+                                                            &to_cset->tasks);
+       }
+}
+
 /*
  * hash table for cgroup groups. This improves the performance to find
  * an existing css_set. This hash doesn't (currently) take into
@@ -2261,47 +2307,6 @@ struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset)
        return NULL;
 }
 
-/**
- * cgroup_task_migrate - move a task from one cgroup to another.
- * @tsk: the task being migrated
- * @new_cset: the new css_set @tsk is being attached to
- *
- * Must be called with cgroup_mutex, threadgroup and css_set_rwsem locked.
- */
-static void cgroup_task_migrate(struct task_struct *tsk,
-                               struct css_set *new_cset)
-{
-       struct css_set *old_cset;
-
-       lockdep_assert_held(&cgroup_mutex);
-       lockdep_assert_held(&css_set_rwsem);
-
-       /*
-        * We are synchronized through cgroup_threadgroup_rwsem against
-        * PF_EXITING setting such that we can't race against cgroup_exit()
-        * changing the css_set to init_css_set and dropping the old one.
-        */
-       WARN_ON_ONCE(tsk->flags & PF_EXITING);
-       old_cset = task_css_set(tsk);
-
-       if (!css_set_populated(new_cset))
-               css_set_update_populated(new_cset, true);
-
-       get_css_set(new_cset);
-       rcu_assign_pointer(tsk->cgroups, new_cset);
-       list_move_tail(&tsk->cg_list, &new_cset->mg_tasks);
-
-       if (!css_set_populated(old_cset))
-               css_set_update_populated(old_cset, false);
-
-       /*
-        * We just gained a reference on old_cset by taking it from the
-        * task. As trading it for new_cset is protected by cgroup_mutex,
-        * we're safe to drop it here; it will be freed under RCU.
-        */
-       put_css_set_locked(old_cset);
-}
-
 /**
  * cgroup_taskset_migrate - migrate a taskset to a cgroup
  * @tset: taget taskset
@@ -2342,8 +2347,14 @@ static int cgroup_taskset_migrate(struct cgroup_taskset *tset,
         */
        down_write(&css_set_rwsem);
        list_for_each_entry(cset, &tset->src_csets, mg_node) {
-               list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list)
-                       cgroup_task_migrate(task, cset->mg_dst_cset);
+               list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list) {
+                       struct css_set *from_cset = task_css_set(task);
+                       struct css_set *to_cset = cset->mg_dst_cset;
+
+                       get_css_set(to_cset);
+                       css_set_move_task(task, from_cset, to_cset, true);
+                       put_css_set_locked(from_cset);
+               }
        }
        up_write(&css_set_rwsem);
 
@@ -5478,9 +5489,8 @@ void cgroup_post_fork(struct task_struct *child,
                down_write(&css_set_rwsem);
                cset = task_css_set(current);
                if (list_empty(&child->cg_list)) {
-                       rcu_assign_pointer(child->cgroups, cset);
-                       list_add_tail(&child->cg_list, &cset->tasks);
                        get_css_set(cset);
+                       css_set_move_task(child, NULL, cset, false);
                }
                up_write(&css_set_rwsem);
        }
@@ -5528,9 +5538,7 @@ void cgroup_exit(struct task_struct *tsk)
 
        if (!list_empty(&tsk->cg_list)) {
                down_write(&css_set_rwsem);
-               list_del_init(&tsk->cg_list);
-               if (!css_set_populated(cset))
-                       css_set_update_populated(cset, false);
+               css_set_move_task(tsk, cset, NULL, false);
                up_write(&css_set_rwsem);
                put_cset = true;
        }