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
---
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
@@ -145,22 +145,40 @@ static void freezer_destroy(struct cgrou
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;
}

@@ -193,12 +211,7 @@ static void check_if_frozen(struct cgrou
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++;
}

@@ -249,11 +262,7 @@ static int try_to_freeze_cgroup(struct c
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++;

--
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