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freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()

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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>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Oleg Nesterov
2012-10-26 19:46:06 +02:00
committed by Tejun Heo
parent ead5c47371
commit 5d8f72b55c
5 changed files with 13 additions and 41 deletions
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13
kernel/power/process.c
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@@ -48,18 +48,7 @@ static int try_to_freeze_tasks(bool user_only)
if (p == current || !freeze_task(p))
continue;
/*
* Now that we've done set_freeze_flag, don't
* perturb a task in TASK_STOPPED or TASK_TRACED.
* It is "frozen enough". If the task does wake
* up, it will immediately call try_to_freeze.
*
* Because freeze_task() goes through p's scheduler lock, it's
* guaranteed that TASK_STOPPED/TRACED -> TASK_RUNNING
* transition can't race with task state testing here.
*/
if (!task_is_stopped_or_traced(p) &&
!freezer_should_skip(p))
if (!freezer_should_skip(p))
todo++;
} while_each_thread(g, p);
read_unlock(&tasklist_lock);