linux-kernel - Re: [PATCH v5 4/7] cgroup: cgroup v2 freezer

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Date:   Tue, 11 Dec 2018 17:26:32 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Roman Gushchin <guroan@...il.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH v5 4/7] cgroup: cgroup v2 freezer

On 12/07, Roman Gushchin wrote:
>
> Cgroup v2 freezer tries to put tasks into a state similar to jobctl
> stop. This means that tasks can be killed, ptraced (using
> PTRACE_SEIZE*), and interrupted. It is possible to attach to
> a frozen task, get some information (e.g. read registers) and detach.

I fail to understand how this all supposed to work.

> @@ -368,6 +369,8 @@ static inline int signal_pending_state(long state, struct task_struct *p)
>  		return 0;
>  	if (!signal_pending(p))
>  		return 0;
> +	if (unlikely(cgroup_task_frozen(p) && p->jobctl == JOBCTL_TRAP_FREEZE))
> +		return __fatal_signal_pending(p);

I think I will never agree with this change ;) and I don't think it actually helps.

> +void cgroup_enter_frozen(void)
> +{
> +	if (!current->frozen) {
> +		spin_lock_irq(&css_set_lock);
> +		current->frozen = true;
> +		cgroup_inc_frozen_cnt(task_dfl_cgroup(current), false, true);
> +		spin_unlock_irq(&css_set_lock);
> +	}
> +
> +	__set_current_state(TASK_INTERRUPTIBLE);
> +	schedule();

So once again, suppose it races with PTRACE_INTERRUPT, or SIGSTOP, or something
else which should be handled by get_signal() before do_freezer_trap().

If (say) PTRACE_INTERRUPT comes before schedule it will be lost. Otherwise
the frozen task will react. This can't be right. Or I am totally confused.

Perhaps you can split this patch? start with cgroup_enter_frozen() using
TASK_KILLABLE, then teach it to handle ptrace/stop/etc? I think this way it
would be simpler to discuss the necessary changes and document what exactly
are you trying to do.

and btw.... what about suspend? try_to_freeze_tasks() will obviously fail
if there is a ->frozen thread?

Oleg.