linux-kernel - Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT

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Message-ID: <Ykwn0MpcrZ/N+LOG@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Apr 2022 13:28:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on
 PREEMPT_RT.

On Tue, Apr 05, 2022 at 12:29:03PM +0200, Oleg Nesterov wrote:
> On 04/05, Peter Zijlstra wrote:
> >
> > As is, I think we can write task_is_stopped() like:
> >
> > #define task_is_stopped(task)	((task)->jobctl & JOBCTL_STOP_PENDING)
> >
> > Because jobctl is in fact the canonical state.
> 
> Not really. JOBCTL_STOP_PENDING means that the task should stop.
> 
> And this flag is cleared right before set_special_state(TASK_STOPPED)
> in do_signal_stop(), see task_participate_group_stop().

Argh! More work then :-( Still, I really want to not have p->__state be
actual unique state.