[PATCH 5/6] Container Freezer: Prevent frozen tasks or cgroups from changing

[Matt Helsley <matthltc@us.ibm.com>]

Don't let frozen tasks or cgroups change. This means frozen tasks can't
leave their current cgroup for another cgroup. It also means that tasks
cannot be added to or removed from a cgroup in the FROZEN state. We
enforce these rules by checking for frozen tasks and cgroups in the
can_attach() function.

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
---
Changes since v4:
v5:
	Checked use of task alloc lock for races with swsusp freeze/thaw --
		looks safe because there are explicit barriers to handle
		freeze/thaw races for individual tasks, we explicitly
		handle partial group freezing, and partial group thawing
		should be resolved without changing swsusp's loop. This should
		answer Li Zefan's last comment re: races between freeze and
		thaw.

 kernel/cgroup_freezer.c |   43 ++++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

Index: linux-2.6.27-rc1-mm1/kernel/cgroup_freezer.c
===================================================================
--- linux-2.6.27-rc1-mm1.orig/kernel/cgroup_freezer.c
+++ linux-2.6.27-rc1-mm1/kernel/cgroup_freezer.c
@@ -90,26 +90,44 @@ static void freezer_destroy(struct cgrou
 			    struct cgroup *cgroup)
 {
 	kfree(cgroup_freezer(cgroup));
 }
 
+/* Task is frozen or will freeze immediately when next it gets woken */
+static bool is_task_frozen_enough(struct task_struct *task)
+{
+	return frozen(task) ||
+		(task_is_stopped_or_traced(task) && freezing(task));
+}
 
+/*
+ * The call to cgroup_lock() in the freezer.state write method prevents
+ * a write to that file racing against an attach, and hence the
+ * can_attach() result will remain valid until the attach completes.
+ */
 static int freezer_can_attach(struct cgroup_subsys *ss,
 			      struct cgroup *new_cgroup,
 			      struct task_struct *task)
 {
 	struct freezer *freezer;
-	int retval = 0;
+	int retval;
+
+	/* Anything frozen can't move or be moved to/from */
+
+	if (is_task_frozen_enough(task))
+		return -EBUSY;
 
-	/*
-	 * The call to cgroup_lock() in the freezer.state write method prevents
-	 * a write to that file racing against an attach, and hence the
-	 * can_attach() result will remain valid until the attach completes.
-	 */
 	freezer = cgroup_freezer(new_cgroup);
 	if (freezer->state == STATE_FROZEN)
+		return -EBUSY;
+
+	retval = 0;
+	task_lock(task);
+	freezer = task_freezer(task);
+	if (freezer->state == STATE_FROZEN)
 		retval = -EBUSY;
+	task_unlock(task);
 	return retval;
 }
 
 static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
 {
@@ -140,16 +158,11 @@ static void check_if_frozen(struct cgrou
 	unsigned int nfrozen = 0, ntotal = 0;
 
 	cgroup_iter_start(cgroup, &it);
 	while ((task = cgroup_iter_next(cgroup, &it))) {
 		ntotal++;
-		/*
-		 * Task is frozen or will freeze immediately when next it gets
-		 * woken
-		 */
-		if (frozen(task) ||
-		    (task_is_stopped_or_traced(task) && freezing(task)))
+		if (is_task_frozen_enough(task))
 			nfrozen++;
 	}
 
 	/*
 	 * Transition to FROZEN when no new tasks can be added ensures
@@ -196,15 +209,11 @@ static int try_to_freeze_cgroup(struct c
 	freezer->state = STATE_FREEZING;
 	cgroup_iter_start(cgroup, &it);
 	while ((task = cgroup_iter_next(cgroup, &it))) {
 		if (!freeze_task(task, true))
 			continue;
-		if (task_is_stopped_or_traced(task) && freezing(task))
-			/*
-			 * The freeze flag is set so these tasks will
-			 * immediately go into the fridge upon waking.
-			 */
+		if (is_task_frozen_enough(task))
 			continue;
 		if (!freezing(task) && !freezer_should_skip(task))
 			num_cant_freeze_now++;
 	}
 	cgroup_iter_end(cgroup, &it);

--