cgroup_freezer: update_freezer_state() does incorrect state transitions

There are 4 state transitions possible for a freezer.  Only FREEZING ->
FROZEN transaction is done lazily.  This patch allows update_freezer_state
only to perform this transaction and renames the function to
update_if_frozen.

Moreover is_task_frozen_enough function is removed and its every occurence
is replaced with frozen().  Therefore for a group to become FROZEN every
task must be frozen.

The previous version could trigger a following bug: When cgroup is in the
process of freezing (but none of its tasks are frozen yet),
update_freezer_state() (called from freezer_read or freezer_write) would
incorrectly report that a group is 'THAWED' (because nfrozen = 0),
allowing the transaction FREEZING -> THAWED without writing anything to
'freezer.state'.  This is incorrect according to the documentation.  This
could result in a 'THAWED' cgroup with frozen tasks inside.

A code to reproduce this bug is available here:
http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug2.c

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Tomasz Buchert <tomasz.buchert@inria.fr>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 23 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index c287627bed3d..e7bebb7c6c38 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -153,13 +153,6 @@ static void freezer_destroy(struct cgroup_subsys *ss,
         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
@@ -236,31 +229,30 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
 /*
  * caller must hold freezer->lock
  */
-static void update_freezer_state(struct cgroup *cgroup,
+static void update_if_frozen(struct cgroup *cgroup,
                                  struct freezer *freezer)
 {
         struct cgroup_iter it;
         struct task_struct *task;
         unsigned int nfrozen = 0, ntotal = 0;
+        enum freezer_state old_state = freezer->state;
 
         cgroup_iter_start(cgroup, &it);
         while ((task = cgroup_iter_next(cgroup, &it))) {
                 ntotal++;
-                if (is_task_frozen_enough(task))
+                if (frozen(task))
                         nfrozen++;
         }
 
-        /*
-         * Transition to FROZEN when no new tasks can be added ensures
-         * that we never exist in the FROZEN state while there are unfrozen
-         * tasks.
-         */
-        if (nfrozen == ntotal)
-                freezer->state = CGROUP_FROZEN;
-        else if (nfrozen > 0)
-                freezer->state = CGROUP_FREEZING;
-        else
-                freezer->state = CGROUP_THAWED;
+        if (old_state == CGROUP_THAWED) {
+                BUG_ON(nfrozen > 0);
+        } else if (old_state == CGROUP_FREEZING) {
+                if (nfrozen == ntotal)
+                        freezer->state = CGROUP_FROZEN;
+        } else { /* old_state == CGROUP_FROZEN */
+                BUG_ON(nfrozen != ntotal);
+        }
+
         cgroup_iter_end(cgroup, &it);
 }
 
@@ -279,7 +271,7 @@ static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
         if (state == CGROUP_FREEZING) {
                 /* We change from FREEZING to FROZEN lazily if the cgroup was
                  * only partially frozen when we exitted write. */
-                update_freezer_state(cgroup, freezer);
+                update_if_frozen(cgroup, freezer);
                 state = freezer->state;
         }
         spin_unlock_irq(&freezer->lock);
@@ -301,7 +293,7 @@ static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
         while ((task = cgroup_iter_next(cgroup, &it))) {
                 if (!freeze_task(task, true))
                         continue;
-                if (is_task_frozen_enough(task))
+                if (frozen(task))
                         continue;
                 if (!freezing(task) && !freezer_should_skip(task))
                         num_cant_freeze_now++;
@@ -335,7 +327,7 @@ static int freezer_change_state(struct cgroup *cgroup,
 
         spin_lock_irq(&freezer->lock);
 
-        update_freezer_state(cgroup, freezer);
+        update_if_frozen(cgroup, freezer);
         if (goal_state == freezer->state)
                 goto out;