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Date:   Wed, 6 Jul 2022 10:59:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sven Schnelle <svens@linux.ibm.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
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        Oleg Nesterov <oleg@redhat.com>, mingo@kernel.org,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        mgorman@suse.de, bigeasy@linutronix.de,
        Will Deacon <will@kernel.org>, tj@kernel.org,
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Subject: Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED,
 TASK_STOPPED state
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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.