3.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Tejun Heo <t...@kernel.org>
commit d87838321124061f6c935069d97f37010fa417e6 upstream.
This reverts commit 7e3aa30ac8c904a706518b725c451bb486daaae9.
The commit incorrectly assumed that fork path always performed
threadgroup_change_begin/end() and depended on that for
synchronization against task exit and cgroup migration paths instead
of explicitly grabbing task_lock().
threadgroup_change is not locked when forking a new process (as
opposed to a new thread in the same process) and even if it were it
wouldn't be effective as different processes use different threadgroup
locks.
Revert the incorrect optimization.
Signed-off-by: Tejun Heo <t...@kernel.org>
LKML-Reference: <20121008020000.GB2575@localhost>
Acked-by: Li Zefan <lize...@huawei.com>
Cc: Frederic Weisbecker <fweis...@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
kernel/cgroup.c | 15 +++------------
1 file changed, 3 insertions(+), 12 deletions(-)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4539,19 +4539,10 @@ void cgroup_post_fork(struct task_struct
*/
if (use_task_css_set_links) {
write_lock(&css_set_lock);
- if (list_empty(&child->cg_list)) {
- /*
- * It's safe to use child->cgroups without task_lock()
- * here because we are protected through
- * threadgroup_change_begin() against concurrent
- * css_set change in cgroup_task_migrate(). Also
- * the task can't exit at that point until
- * wake_up_new_task() is called, so we are protected
- * against cgroup_exit() setting child->cgroup to
- * init_css_set.
- */
+ task_lock(child);
+ if (list_empty(&child->cg_list))
list_add(&child->cg_list, &child->cgroups->tasks);
- }
+ task_unlock(child);
write_unlock(&css_set_lock);
}
}
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