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Commit 9bb71308 authored by Tejun Heo's avatar Tejun Heo
Browse files

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



This reverts commit 7e381b0e.

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: default avatarTejun Heo <tj@kernel.org>
LKML-Reference: <20121008020000.GB2575@localhost>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
Bitterly-Acked-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
Cc: stable@vger.kernel.org
parent 1f5320d5

kernel/cgroup.c

+6 −17
Original line number Diff line number Diff line
@@ -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);

}