Re: [PATCH v10 4/9] cgroup: cgroup v2 freezer

[Oleg Nesterov]

On 04/05, Roman Gushchin wrote:
>
> @@ -5830,6 +5926,12 @@ void cgroup_exit(struct task_struct *tsk)
>               spin_lock_irq(&css_set_lock);
>               css_set_move_task(tsk, cset, NULL, false);
>               cset->nr_tasks--;
> +
> +             if (unlikely(cgroup_task_frozen(tsk)))
> +                     cgroup_freezer_frozen_exit(tsk);
> +             else if (unlikely(cgroup_task_freeze(tsk)))
> +                     cgroup_update_frozen(task_dfl_cgroup(tsk));
> +

Now that I actually applied these patches, I can't understand how can
cgroup_exit() hit cgroup_task_frozen(current)...

A cgroup_task_frozen() task must never return to user-mode or escape
from get_signal() without leave_frozen() or we have a bug?

I must have missed something, could you explain why the patch below is
wrong?

Oleg.


diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 3e2efd4..24b8757 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -889,7 +889,6 @@ void cgroup_update_frozen(struct cgroup *cgrp);
 void cgroup_freeze(struct cgroup *cgrp, bool freeze);
 void cgroup_freezer_migrate_task(struct task_struct *task, struct cgroup *src,
                                 struct cgroup *dst);
-void cgroup_freezer_frozen_exit(struct task_struct *task);
 static inline bool cgroup_task_freeze(struct task_struct *task)
 {
        bool ret;
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6f09f9b..3d5f76a 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6008,11 +6008,8 @@ void cgroup_exit(struct task_struct *tsk)
                css_set_move_task(tsk, cset, NULL, false);
                cset->nr_tasks--;
 
-               if (unlikely(cgroup_task_frozen(tsk)))
-                       cgroup_freezer_frozen_exit(tsk);
-               else if (unlikely(cgroup_task_freeze(tsk)))
+               if (unlikely(cgroup_task_freeze(tsk)))
                        cgroup_update_frozen(task_dfl_cgroup(tsk));
-
                spin_unlock_irq(&css_set_lock);
        } else {
                get_css_set(cset);
diff --git a/kernel/cgroup/freezer.c b/kernel/cgroup/freezer.c
index b5ccc87..68aa506 100644
--- a/kernel/cgroup/freezer.c
+++ b/kernel/cgroup/freezer.c
@@ -249,16 +249,6 @@ void cgroup_freezer_migrate_task(struct task_struct *task,
        cgroup_freeze_task(task, test_bit(CGRP_FREEZE, &dst->flags));
 }
 
-void cgroup_freezer_frozen_exit(struct task_struct *task)
-{
-       struct cgroup *cgrp = task_dfl_cgroup(task);
-
-       lockdep_assert_held(&css_set_lock);
-
-       cgroup_dec_frozen_cnt(cgrp);
-       cgroup_update_frozen(cgrp);
-}
-
 void cgroup_freeze(struct cgroup *cgrp, bool freeze)
 {
        struct cgroup_subsys_state *css;