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Revert "cgroup: Drop task_lock(parent) on cgroup_fork()"

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This reverts commit 7e381b0eb1.

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>
Bitterly-Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: stable@vger.kernel.org
This commit is contained in:
Tejun Heo 2012-10-18 17:52:07 -07:00
parent 1f5320d597
commit 9bb71308b8
1 changed files with 6 additions and 17 deletions
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23
kernel/cgroup.c
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@ -4814,31 +4814,20 @@ static const struct file_operations proc_cgroupstats_operations = {
*
* A pointer to the shared css_set was automatically copied in
* fork.c by dup_task_struct(). However, we ignore that copy, since
* it was not made under the protection of RCU, cgroup_mutex or
* threadgroup_change_begin(), so it might no longer be a valid
* cgroup pointer. cgroup_attach_task() might have already changed
* current->cgroups, allowing the previously referenced cgroup
* group to be removed and freed.
*
* Outside the pointer validity we also need to process the css_set
* inheritance between threadgoup_change_begin() and
* threadgoup_change_end(), this way there is no leak in any process
* wide migration performed by cgroup_attach_proc() that could otherwise
* miss a thread because it is too early or too late in the fork stage.
* it was not made under the protection of RCU or cgroup_mutex, so
* might no longer be a valid cgroup pointer. cgroup_attach_task() might
* have already changed current->cgroups, allowing the previously
* referenced cgroup group to be removed and freed.
*
* At the point that cgroup_fork() is called, 'current' is the parent
* task, and the passed argument 'child' points to the child task.
*/
void cgroup_fork(struct task_struct *child)
{
/*
* We don't need to task_lock() current because current->cgroups
* can't be changed concurrently here. The parent obviously hasn't
* exited and called cgroup_exit(), and we are synchronized against
* cgroup migration through threadgroup_change_begin().
*/
task_lock(current);
child->cgroups = current->cgroups;
get_css_set(child->cgroups);
task_unlock(current);
INIT_LIST_HEAD(&child->cg_list);
}