linux-kernel - Re: [PATCH] sched: Fix race against ptrace_freeze

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Message-ID: <20200721153823.GA28364@redhat.com>
Date:   Tue, 21 Jul 2020 17:38:24 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     peterz@...radead.org
Cc:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        christian@...uner.io, "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>,
        Dave Jones <davej@...emonkey.org.uk>
Subject: Re: [PATCH] sched: Fix race against ptrace_freeze_trace()

On 07/21, Peter Zijlstra wrote:
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4193,9 +4193,6 @@ static void __sched notrace __schedule(b
>  	local_irq_disable();
>  	rcu_note_context_switch(preempt);
>
> -	/* See deactivate_task() below. */
> -	prev_state = prev->state;
> -
>  	/*
>  	 * Make sure that signal_pending_state()->signal_pending() below
>  	 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> @@ -4219,11 +4216,16 @@ static void __sched notrace __schedule(b
>  	update_rq_clock(rq);
>
>  	switch_count = &prev->nivcsw;
> +
>  	/*
> -	 * We must re-load prev->state in case ttwu_remote() changed it
> -	 * before we acquired rq->lock.
> +	 * We must load prev->state once (task_struct::state is volatile), such
> +	 * that:
> +	 *
> +	 *  - we form a control dependency vs deactivate_task() below.
> +	 *  - ptrace_{,un}freeze_traced() can change ->state underneath us.
>  	 */
> -	if (!preempt && prev_state && prev_state == prev->state) {
> +	prev_state = prev->state;
> +	if (!preempt && prev_state) {

Thanks! FWIW,

Acked-by: Oleg Nesterov <oleg@...hat.com>