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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 <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
This commit is contained in:
Oleg Nesterov 2012-10-26 19:46:06 +02:00 • committed by Tejun Heo
commit 5d8f72b55c
5 changed files with 13 additions and 41 deletions
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11
kernel/freezer.c
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@ -116,17 +116,10 @@ bool freeze_task(struct task_struct *p)
return false;
}
if (!(p->flags & PF_KTHREAD)) {
if (!(p->flags & PF_KTHREAD))
fake_signal_wake_up(p);
/*
* fake_signal_wake_up() goes through p's scheduler
* lock and guarantees that TASK_STOPPED/TRACED ->
* TASK_RUNNING transition can't race with task state
* testing in try_to_freeze_tasks().
*/
} else {
else
wake_up_state(p, TASK_INTERRUPTIBLE);
}
spin_unlock_irqrestore(&freezer_lock, flags);
return true;