Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state

Peter Zijlstra
2022-07-06T08:59:00Z

On Wed, Jul 06, 2022 at 09:58:55AM +0200, Sven Schnelle wrote:

> >> [ 86.218551] kill_chi-343805 6d.... 79990141us : ptrace_stop: JOBCTL_TRACED already set, state=0 <------ valid combination of flags?
> >
> > Yeah, that's not supposed to be so. JOBCTL_TRACED is supposed to follow
> > __TASK_TRACED for now. Set when __TASK_TRACED, cleared when
> > TASK_RUNNING.
> >
> > Specifically {ptrace_,}signal_wake_up() in signal.h clear JOBCTL_TRACED
> > when they would wake a __TASK_TRACED task.
>
> try_to_wake_up() clears TASK_TRACED in this case because a signal
> (SIGKILL) has to be delivered. As a test I put the following change
> on top, and it "fixes" the problem:
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index da0bf6fe9ecd..f2e0f5e70e77 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4141,6 +4149,9 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> * TASK_WAKING such that we can unlock p->pi_lock before doing the
> * enqueue, such as ttwu_queue_wakelist().
> */
> + if (p->__state & TASK_TRACED)
> + trace_printk("clearing TASK_TRACED 2\n");
> + p->jobctl &= ~JOBCTL_TRACED;
> WRITE_ONCE(p->__state, TASK_WAKING);
>
> /*
>
> There are several places where the state is changed from TASK_TRACED to
> something else without clearing JOBCTL_TRACED.

I'm having difficulty spotting them; I find:

TASK_WAKEKILL: signal_wake_up()
__TASK_TRACED: ptrace_signal_wake_up(), ptrace_unfreeze_traced(), ptrace_resume()

And all those sites dutifully clear JOBCTL_TRACED.

I'd be most interested in the calstack for the 'clearing TASK_TRACED 2'
events to see where we miss a spot.