linux-kernel - Re: [PATCH v6 4/7] cgroup: cgroup v2 freezer

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Date:   Fri, 25 Jan 2019 13:27:13 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Roman Gushchin <guroan@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, kernel-team@...com,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH v6 4/7] cgroup: cgroup v2 freezer

Sorry, this version raced with my vacation, I missed it.

I'll try to read this code carefully but after a quick glance I have some
concerns,

On 12/21, Roman Gushchin wrote:
>
> +static void cgroup_update_frozen(struct cgroup *cgrp)
> +{
> +	bool frozen;
> +
> +	lockdep_assert_held(&css_set_lock);
> +
> +	/*
> +	 * If the cgroup has to be frozen (CGRP_FREEZE bit set),
> +	 * and all tasks are frozen or stopped, let's consider
> +	 * the cgroup frozen. Otherwise it's not frozen.
> +	 */
> +	frozen = test_bit(CGRP_FREEZE, &cgrp->flags) &&
> +		cgrp->freezer.nr_frozen_tasks +
> +		cgrp->freezer.nr_stopped_tasks ==
> +		cgrp->freezer.nr_tasks_to_freeze;

OK. Suppose that cgroup is frozen, CGRP_FROZEN is set, stopped == 0,
to_freeze = frozen.

One of the task is killed, it calls leave_frozen(). If I read this code path
correctly, only ->nr_frozen_tasks will be decremented, so "frozen" will be
"false" when cgroup_update_frozen() is called.

Doesn't this mean that this cgroup will no longer be CGRP_FROZEN even after
the killed task goes away completely?

Or. Suppose that another process picks a task from the CGRP_FROZEN cgroup and
does PTRACE_ATTACH + PTRACE_INTERRUPT. IIUC, the tracee will only increment
->nr_stopped_tasks, it won't touch other counters. Again, cgroup won't be FROZEN
until PTRACE_CONT'ed tracee does cgroup_leave_stopped() ? This looks strange at
least.

SIGSTOP. IIUC, a frozen task sleeping in do_freezer_trap() won't stop. However if
another thread has already called do_signal_stop(), the woken frozen task will
react to JOBCTL_STOP_PENDING and stop. And do_signal_stop()->cgroup_enter_stopped()
will "destroy" CGRP_FROZEN too, or I am totally confused.

OTOH, if you freeze a TASK_STOPPED task's cgroup, this task can react to SIGCONT,
notify its parent, then freeze again. This is fine, but iiuc this cgroup won't be
FROZEN in between, cgroup_file_notify() will be called twice...

Oleg.