linux-kernel - Re: [PATCH v2 1/5] sched,signal,ptrace: Rework TASK_TRACED, TASK

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Date:   Thu, 28 Apr 2022 12:00:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     rjw@...ysocki.net, oleg@...hat.com, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 1/5] sched,signal,ptrace: Rework TASK_TRACED,
 TASK_STOPPED state

On Tue, Apr 26, 2022 at 06:34:09PM -0500, Eric W. Biederman wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > Currently ptrace_stop() / do_signal_stop() rely on the special states
> > TASK_TRACED and TASK_STOPPED resp. to keep unique state. That is, this
> > state exists only in task->__state and nowhere else.
> >
> > There's two spots of bother with this:
> >
> >  - PREEMPT_RT has task->saved_state which complicates matters,
> >    meaning task_is_{traced,stopped}() needs to check an additional
> >    variable.
> >
> >  - An alternative freezer implementation that itself relies on a
> >    special TASK state would loose TASK_TRACED/TASK_STOPPED and will
> >    result in misbehaviour.
> >
> > As such, add additional state to task->jobctl to track this state
> > outside of task->__state.
> >
> > NOTE: this doesn't actually fix anything yet, just adds extra state.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -770,7 +773,9 @@ void signal_wake_up_state(struct task_st
> >  	 * By using wake_up_state, we ensure the process will wake up and
> >  	 * handle its death signal.
> >  	 */
> > -	if (!wake_up_state(t, state | TASK_INTERRUPTIBLE))
> > +	if (wake_up_state(t, state | TASK_INTERRUPTIBLE))
> > +		t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
> > +	else
> >  		kick_process(t);
> >  }
> 
> This hunk is subtle and I don't think it is actually what we want if the
> code is going to be robust against tsk->__state becoming TASK_FROZEN.

Oooh, indeed. Yes, let me go back to that resume based thing as you
suggest.

But first, let me go read all your patches :-)