linux-kernel - Re: [PATCH 7/9] ptrace: Simplify the wait_task_inactive call in ptrace_check

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Date:   Thu, 28 Apr 2022 18:19:13 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        inux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 7/9] ptrace: Simplify the wait_task_inactive call in
 ptrace_check_attach

On 04/28, Peter Zijlstra wrote:
>
> On Thu, Apr 28, 2022 at 04:57:50PM +0200, Oleg Nesterov wrote:
>
> > > Shouldn't we then switch wait_task_inactive() so have & matching instead
> > > of the current ==.
> >
> > Sorry, I don't understand the context...
>
> This.. I've always found it strange to have wti use a different matching
> scheme from ttwu.

Ah. This is what I understood (and I too thought about this), just I meant that
this patch from Eric (assuming wait_task_inactive() still uses __TASK_TRACED) is
fine without your change below.

Oleg.

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f259621f4c93..c039aef4c8fe 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3304,7 +3304,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
>  		 * is actually now running somewhere else!
>  		 */
>  		while (task_running(rq, p)) {
> -			if (match_state && unlikely(READ_ONCE(p->__state) != match_state))
> +			if (match_state && unlikely(!(READ_ONCE(p->__state) & match_state)))
>  				return 0;
>  			cpu_relax();
>  		}
> @@ -3319,7 +3319,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
>  		running = task_running(rq, p);
>  		queued = task_on_rq_queued(p);
>  		ncsw = 0;
> -		if (!match_state || READ_ONCE(p->__state) == match_state)
> +		if (!match_state || (READ_ONCE(p->__state) & match_state))
>  			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
>  		task_rq_unlock(rq, p, &rf);