[PATCH 13/16] cgroup_freezer: prepare for removal of TIF_FREEZE

[Tejun Heo <tj@kernel.org>]

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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
---
 include/linux/freezer.h |    6 +++---
 kernel/cgroup_freezer.c |   36 ++++++++++++------------------------
 kernel/power/process.c  |    2 +-
 3 files changed, 16 insertions(+), 28 deletions(-)

diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 13559a8..bcc0637 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -63,11 +63,11 @@ extern bool freeze_task(struct task_struct *p, bool sig_only);
 extern bool __set_freezable(bool with_signal);
 
 #ifdef CONFIG_CGROUP_FREEZER
-extern int cgroup_freezing_or_frozen(struct task_struct *task);
+extern bool cgroup_freezing(struct task_struct *task);
 #else /* !CONFIG_CGROUP_FREEZER */
-static inline int cgroup_freezing_or_frozen(struct task_struct *task)
+static inline bool cgroup_freezing(struct task_struct *task)
 {
-	return 0;
+	return false;
 }
 #endif /* !CONFIG_CGROUP_FREEZER */
 
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index e7fa0ec..0478c35 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,
@@ -321,6 +307,8 @@ static int freezer_change_state(struct cgroup *cgroup,
 	if (goal_state == freezer->state)
 		goto out;
 
+	freezer->state = goal_state;
+
 	switch (goal_state) {
 	case CGROUP_THAWED:
 		unfreeze_cgroup(cgroup, freezer);
diff --git a/kernel/power/process.c b/kernel/power/process.c
index f8012f2..1bbc363 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -154,7 +154,7 @@ void thaw_processes(void)
 
 	read_lock(&tasklist_lock);
 	do_each_thread(g, p) {
-		if (cgroup_freezing_or_frozen(p))
+		if (cgroup_freezing(p))
 			continue;
 
 		__thaw_task(p);
-- 
1.7.6