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

From: Oleg Nesterov
Date: Tue Dec 11 2018 - 11:26:39 EST

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?