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

[Roman Gushchin <guro@fb.com>]

On Tue, Dec 11, 2018 at 05:26:32PM +0100, Oleg Nesterov wrote:
> 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.

See below.

> 
> > +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.

Why?
PTRACE_INTERRUPT will set JOBCTL_TRAP_STOP, so signal_pending_state()
will return true, schedule() will return immediately, and we'll handle the trap.

> 
> 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?

I have to think a bit more here, but something like this will probably work:

diff --git a/kernel/freezer.c b/kernel/freezer.c
index b162b74611e4..590ac4d10b02 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -134,7 +134,7 @@ bool freeze_task(struct task_struct *p)
                return false;
 
        spin_lock_irqsave(&freezer_lock, flags);
-       if (!freezing(p) || frozen(p)) {
+       if (!freezing(p) || frozen(p) || cgroup_task_frozen()) {
                spin_unlock_irqrestore(&freezer_lock, flags);
                return false;
        }

--

If the task is already frozen by the cgroup freezer, we don't have to do
anything additionally.

Thanks!