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

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Date:   Mon, 14 Mar 2022 19:54:30 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT.

I never really understood ->saved_state logic. Will read this patch
tomorrow, but at first glance this patch doesn't solve all problems.

On 03/02, Sebastian Andrzej Siewior wrote:
>
> +static inline bool __task_state_match_eq(struct task_struct *tsk, long state)
> +{
> +	bool match = false;
> +
> +	if (READ_ONCE(tsk->__state) == state)
> +		match = true;
> +	else if (tsk->saved_state == state)
> +		match = true;
> +	return match;
> +}

...

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3239,7 +3239,8 @@ 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(!task_state_match_eq(p, match_state)))
>  				return 0;

So wait_task_inactive() can return 0 but the task can run after that, right?
This is not what we want...

Oleg.