2012/10/19 Tejun Heo [off-list ref]:

On Fri, Oct 19, 2012 at 09:35:26AM -0400, Frederic Weisbecker wrote:

quoted

2012/10/18 Tejun Heo [off-list ref]:

quoted

From d935a5d6832a264ce52f4257e176f4f96cbaf048 Mon Sep 17 00:00:00 2001
From: Tejun Heo <redacted>
Date: Thu, 18 Oct 2012 17:40:30 -0700

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.

Ok but there is still no good reason to task_lock() there. But the
comment is indeed wrong,  how about fixing it instead? I can send you
a patch for that.

For -stable, I think it's better to revert.  If you want to remove
task_lock, let's do it for 3.8.

I don't think that a wrong comment justifies a patch to stable.