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Revert "cgroup: Remove task_lock() from cgroup_post_fork()"

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This reverts commit 7e3aa30ac8.

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 <tj@kernel.org>
LKML-Reference: <20121008020000.GB2575@localhost>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: stable@vger.kernel.org
This commit is contained in:
Tejun Heo 2012-10-18 17:40:30 -07:00
parent 9bb71308b8
commit d878383211
1 changed files with 3 additions and 12 deletions
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15
kernel/cgroup.c
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@ -4883,19 +4883,10 @@ void cgroup_post_fork(struct task_struct *child)
*/
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);
}
}