cgroup_freezer: prepare for removal of TIF_FREEZE

TIF_FREEZE will be removed soon and freezing() will directly test
whether any freezing condition is in effect.  Make the following
changes in preparation.

* Rename cgroup_freezing_or_frozen() to cgroup_freezing() and make it
  return bool.

* Make cgroup_freezing() access task_freezer() under rcu read lock
  instead of task_lock().  This makes the state dereferencing racy
  against task moving to another cgroup; however, it was already racy
  without this change as ->state dereference wasn't synchronized.
  This will be later dealt with using attach hooks.

* freezer->state is now set before trying to push tasks into the
  target state.

-v2: Oleg pointed out that freeze_change_state() was setting
     freeze->state incorrectly to CGROUP_FROZEN instead of
     CGROUP_FREEZING.  Fixed.

-v3: Matt pointed out that setting CGROUP_FROZEN used to always invoke
     try_to_freeze_cgroup() regardless of the current state.  Patch
     updated such that the actual freeze/thaw operations are always
     performed on invocation.  This shouldn't make any difference
     unless something is broken.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Paul Menage <paul@paulmenage.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 13 insertions, 27 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index cd27b0825560..e6a1b8d1b8bc 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -48,19 +48,17 @@ static inline struct freezer *task_freezer(struct task_struct *task)
                             struct freezer, css);
 }
 
-static inline int __cgroup_freezing_or_frozen(struct task_struct *task)
+bool cgroup_freezing(struct task_struct *task)
 {
-        enum freezer_state state = task_freezer(task)->state;
-        return (state == CGROUP_FREEZING) || (state == CGROUP_FROZEN);
-}
+        enum freezer_state state;
+        bool ret;
 
-int cgroup_freezing_or_frozen(struct task_struct *task)
-{
-        int result;
-        task_lock(task);
-        result = __cgroup_freezing_or_frozen(task);
-        task_unlock(task);
-        return result;
+        rcu_read_lock();
+        state = task_freezer(task)->state;
+        ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
+        rcu_read_unlock();
+
+        return ret;
 }
 
 /*
@@ -102,9 +100,6 @@ struct cgroup_subsys freezer_subsys;
  * freezer_can_attach():
  * cgroup_mutex (held by caller of can_attach)
  *
- * cgroup_freezing_or_frozen():
- * task->alloc_lock (to get task's cgroup)
- *
  * freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
  * freezer->lock
  *  sighand->siglock (if the cgroup is freezing)
@@ -177,13 +172,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
 
 static int freezer_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
-        rcu_read_lock();
-        if (__cgroup_freezing_or_frozen(tsk)) {
-                rcu_read_unlock();
-                return -EBUSY;
-        }
-        rcu_read_unlock();
-        return 0;
+        return cgroup_freezing(tsk) ? -EBUSY : 0;
 }
 
 static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
@@ -279,7 +268,6 @@ static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
         struct task_struct *task;
         unsigned int num_cant_freeze_now = 0;
 
-        freezer->state = CGROUP_FREEZING;
         cgroup_iter_start(cgroup, &it);
         while ((task = cgroup_iter_next(cgroup, &it))) {
                 if (!freeze_task(task, true))
@@ -303,8 +291,6 @@ static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
         while ((task = cgroup_iter_next(cgroup, &it)))
                 __thaw_task(task);
         cgroup_iter_end(cgroup, &it);
-
-        freezer->state = CGROUP_THAWED;
 }
 
 static int freezer_change_state(struct cgroup *cgroup,
@@ -318,20 +304,20 @@ static int freezer_change_state(struct cgroup *cgroup,
         spin_lock_irq(&freezer->lock);
 
         update_if_frozen(cgroup, freezer);
-        if (goal_state == freezer->state)
-                goto out;
 
         switch (goal_state) {
         case CGROUP_THAWED:
+                freezer->state = CGROUP_THAWED;
                 unfreeze_cgroup(cgroup, freezer);
                 break;
         case CGROUP_FROZEN:
+                freezer->state = CGROUP_FREEZING;
                 retval = try_to_freeze_cgroup(cgroup, freezer);
                 break;
         default:
                 BUG();
         }
-out:
+
         spin_unlock_irq(&freezer->lock);
 
         return retval;