linux-kernel - Re: [PATCH] freezer,sched: Rewrite core freezer logic

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Message-ID: <20210614154246.GB13677@redhat.com>
Date:   Mon, 14 Jun 2021 17:42:47 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        Will Deacon <will@...nel.org>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] freezer,sched: Rewrite core freezer logic

Hi Peter, sorry for delay,

On 06/11, Peter Zijlstra wrote:
>
> +/* Recursion relies on tail-call optimization to not blow away the stack */
> +static bool __frozen(struct task_struct *p)
> +{
> +	if (p->state == TASK_FROZEN)
> +		return true;
> +
> +	/*
> +	 * If stuck in TRACED, and the ptracer is FROZEN, we're frozen too.
> +	 */
> +	if (task_is_traced(p))
> +		return frozen(rcu_dereference(p->parent));

Why does it use frozen(), not __frozen() ?

This looks racy, p->parent can resume this task and then enter
__refrigerator().

Plus this task can be SIGKILL'ed even if it is traced.

> +	/*
> +	 * If stuck in STOPPED and the parent is FROZEN, we're frozen too.
> +	 */
> +	if (task_is_stopped(p))
> +		return frozen(rcu_dereference(p->real_parent));

(you could use ->parent in this case too and unify this check with the
"traced" case above)

I don't understand. How this connects to ->parent or ->real_parent?
SIGCONT can come from anywhere and wake this stopped task up?

I guess you do this to avoid freezable_schedule() in ptrace/signal_stop,
and we can't use TASK_STOPPED|TASK_FREEZABLE, it should not run after
thaw()... But see above, we can't rely on __frozen(parent).

Oleg.