cgroup: fix RCU accesses around task->cgroups

There are several places in kernel/cgroup.c where task->cgroups is
accessed and modified without going through proper RCU accessors.
None is broken as they're all lock protected accesses; however, this
still triggers sparse RCU address space warnings.

* Consistently use task_css_set() for task->cgroups dereferencing.

* Use RCU_INIT_POINTER() to clear task->cgroups to &init_css_set on
  exit.

* Remove unnecessary rcu_dereference_raw() from cset->subsys[]
  dereference in cgroup_exit().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -724,7 +724,7 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task,
 	 * task can't change groups, so the only thing that can happen
 	 * is that it exits and its css is set back to init_css_set.
 	 */
-	cset = task->cgroups;
+	cset = task_css_set(task);
 	if (cset == &init_css_set) {
 		res = &root->top_cgroup;
 	} else {
@@ -1971,7 +1971,7 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
 	 * css_set to init_css_set and dropping the old one.
 	 */
 	WARN_ON_ONCE(tsk->flags & PF_EXITING);
-	old_cset = tsk->cgroups;
+	old_cset = task_css_set(tsk);
 
 	task_lock(tsk);
 	rcu_assign_pointer(tsk->cgroups, new_cset);
@@ -2094,8 +2094,11 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
 	 * we use find_css_set, which allocates a new one if necessary.
 	 */
 	for (i = 0; i < group_size; i++) {
+		struct css_set *old_cset;
+
 		tc = flex_array_get(group, i);
-		tc->cg = find_css_set(tc->task->cgroups, cgrp);
+		old_cset = task_css_set(tc->task);
+		tc->cg = find_css_set(old_cset, cgrp);
 		if (!tc->cg) {
 			retval = -ENOMEM;
 			goto out_put_css_set_refs;
@@ -3012,7 +3015,7 @@ static void cgroup_enable_task_cg_lists(void)
 		 * entry won't be deleted though the process has exited.
 		 */
 		if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))
-			list_add(&p->cg_list, &p->cgroups->tasks);
+			list_add(&p->cg_list, &task_css_set(p)->tasks);
 		task_unlock(p);
 	} while_each_thread(g, p);
 	read_unlock(&tasklist_lock);
@@ -5061,8 +5064,8 @@ static const struct file_operations proc_cgroupstats_operations = {
 void cgroup_fork(struct task_struct *child)
 {
 	task_lock(current);
+	get_css_set(task_css_set(current));
 	child->cgroups = current->cgroups;
-	get_css_set(child->cgroups);
 	task_unlock(current);
 	INIT_LIST_HEAD(&child->cg_list);
 }
@@ -5097,7 +5100,7 @@ void cgroup_post_fork(struct task_struct *child)
 		write_lock(&css_set_lock);
 		task_lock(child);
 		if (list_empty(&child->cg_list))
-			list_add(&child->cg_list, &child->cgroups->tasks);
+			list_add(&child->cg_list, &task_css_set(child)->tasks);
 		task_unlock(child);
 		write_unlock(&css_set_lock);
 	}
@@ -5177,8 +5180,8 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 
 	/* Reassign the task to the init_css_set. */
 	task_lock(tsk);
-	cset = tsk->cgroups;
-	tsk->cgroups = &init_css_set;
+	cset = task_css_set(tsk);
+	RCU_INIT_POINTER(tsk->cgroups, &init_css_set);
 
 	if (run_callbacks && need_forkexit_callback) {
 		/*
@@ -5187,8 +5190,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 		 */
 		for_each_builtin_subsys(ss, i) {
 			if (ss->exit) {
-				struct cgroup *old_cgrp =
-					rcu_dereference_raw(cset->subsys[i])->cgroup;
+				struct cgroup *old_cgrp = cset->subsys[i]->cgroup;
 				struct cgroup *cgrp = task_cgroup(tsk, i);
 
 				ss->exit(cgrp, old_cgrp, tsk);
@@ -5555,7 +5557,7 @@ static u64 current_css_set_refcount_read(struct cgroup *cgrp,
 	u64 count;
 
 	rcu_read_lock();
-	count = atomic_read(&current->cgroups->refcount);
+	count = atomic_read(&task_css_set(current)->refcount);
 	rcu_read_unlock();
 	return count;
 }