linux-kernel - Re: [PATCH v2 4/4] freezer,sched: Rewrite core freezer logic

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Date:   Wed, 7 Jul 2021 16:14:12 +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>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 4/4] freezer,sched: Rewrite core freezer logic

sorry for delay...

I am still trying to understand this series, just one note for now.

On 06/24, Peter Zijlstra wrote:
>
> +static bool __freeze_task(struct task_struct *p)
> +{
> +	unsigned long flags;
> +	unsigned int state;
> +	bool frozen = false;
> +
> +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> +	state = READ_ONCE(p->__state);
> +	if (state & (TASK_FREEZABLE|__TASK_STOPPED|__TASK_TRACED)) {
> +		/*
> +		 * Only TASK_NORMAL can be augmented with TASK_FREEZABLE,
> +		 * since they can suffer spurious wakeups.
> +		 */
> +		if (state & TASK_FREEZABLE)
> +			WARN_ON_ONCE(!(state & TASK_NORMAL));
> +
> +#ifdef CONFIG_LOCKDEP
> +		/*
> +		 * It's dangerous to freeze with locks held; there be dragons there.
> +		 */
> +		if (!(state & __TASK_FREEZABLE_UNSAFE))
> +			WARN_ON_ONCE(debug_locks && p->lockdep_depth);
> +#endif
> +
> +		if (state & (__TASK_STOPPED|__TASK_TRACED))
> +			WRITE_ONCE(p->__state, TASK_FROZEN|__TASK_FROZEN_SPECIAL);

Well, this doesn't look right.

Firstly, this can race with ptrace_freeze_traced() which can set
p->__state = __TASK_TRACED and clear TASK_FROZEN. Or with
__set_current_state(TASK_RUNNING) in ptrace_stop().

But the main problem is that you can't simply remove __TASK_TRACED,
this can confuse the debugger, any ptrace() request will fail as if
the tracee was killed.

Another problem. Suppose that p->parent sleeps in do_wait(). p calls
ptrace_stop(), sets __TASK_TRACED, and wakes the parent up.

__freeze_task() clears __TASK_TRACED.

The parent calls wait_task_stopped(p) but it fails because
task_is_traced() returns false. The parent sleeps again, and forever
because __thaw_special() won't notify it.

Or. Suppose that __freeze_task() removes __TASK_STOPPED. The new
debugger comes, the tracee should switch from STOPPED to TRACED. But
this won't happen because task_is_stopped() in ptrace_() will return
false and task_set_jobctl_pending/signal_wake_up_state won't be called.

Oleg.