On Fri, 2012-10-26 at 19:46 +0200, Oleg Nesterov wrote:
> try_to_freeze_tasks() and cgroup_freezer rely on scheduler locks
> to ensure that a task doing STOPPED/TRACED -> RUNNING transition
> can't escape freezing. This mostly works, but ptrace_stop() does
> not necessarily call schedule(), it can change task->state back to
> RUNNING and check freezing() without any lock/barrier in between.
> 
> We could add the necessary barrier, but this patch changes
> ptrace_stop() and do_signal_stop() to use freezable_schedule().
> This fixes the race, freezer_count() and freezer_should_skip()
> carefully avoid the race.
> 
> And this simplifies the code, try_to_freeze_tasks/update_if_frozen
> no longer need to use task_is_stopped_or_traced() checks with the
> non trivial assumptions. We can rely on the mechanism which was
> specially designed to mark the sleeping task as "frozen enough".
> 
> v2: As Tejun pointed out, we can also change get_signal_to_deliver()
> and move try_to_freeze() up before 'relock' label.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[...]

This is not the correct way to submit a change to stable.  Please see
Documentation/stable_kernel_rules.txt

Ben.

-- 
Ben Hutchings
Never attribute to conspiracy what can adequately be explained by stupidity.