Re: [PATCH] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT.

From: Oleg Nesterov <oleg@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [PATCH] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT.
Date: Mon, 14 Mar 2022 19:54:30 +0100	[thread overview]
Message-ID: <20220314185429.GA30364@redhat.com> (raw)
In-Reply-To: <Yh/b19JikC+Vnm8i@linutronix.de>

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.