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cgroup_freezer: fix can_attach() to prohibit moving from/to freezing/frozen cgroups

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It is possible to move a task from its cgroup even if this group is
'FREEZING'.  This results in a nasty bug - the moved task will become
frozen OUTSIDE its original cgroup and will remain in a permanent 'D'
state.

This patch allows to migrate the task only between THAWED cgroups.

This behavior was observed and easily reproduced on a single core laptop.
Notice that reproducibility depends highly on the machine used.  Program
and instructions how to reproduce the bug can be fetched from:
http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug.c

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>
This commit is contained in:
Tomasz Buchert 2010-10-27 15:33:33 -07:00 committed by Linus Torvalds
parent d5de4ddb1b
commit 0bdba580ab
1 changed files with 8 additions and 7 deletions
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15
kernel/cgroup_freezer.c
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@ -173,24 +173,25 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
/*
* Anything frozen can't move or be moved to/from.
*
* Since orig_freezer->state == FROZEN means that @task has been
* frozen, so it's sufficient to check the latter condition.
*/
if (is_task_frozen_enough(task))
freezer = cgroup_freezer(new_cgroup);
if (freezer->state != CGROUP_THAWED)
return -EBUSY;
freezer = cgroup_freezer(new_cgroup);
if (freezer->state == CGROUP_FROZEN)
rcu_read_lock();
if (__cgroup_freezing_or_frozen(task)) {
rcu_read_unlock();
return -EBUSY;
}
rcu_read_unlock();
if (threadgroup) {
struct task_struct *c;
rcu_read_lock();
list_for_each_entry_rcu(c, &task->thread_group, thread_group) {
if (is_task_frozen_enough(c)) {
if (__cgroup_freezing_or_frozen(c)) {
rcu_read_unlock();
return -EBUSY;
}