linux-kernel - Re: [PATCH v10 4/9] cgroup: cgroup v2 freezer

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Message-ID: <20190419160806.GA12790@redhat.com>
Date:   Fri, 19 Apr 2019 18:08:06 +0200
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 v10 4/9] cgroup: cgroup v2 freezer

Anyway, I do not want to delay this feature. Even if I am right we can
cleanup this code later. I see nothing really wrong, so no objections
from me. Sorry for delay.


On 04/19, Oleg Nesterov wrote:
>
> On 04/05, Roman Gushchin wrote:
> >
> > +void cgroup_leave_frozen(bool always_leave)
> > +{
> > +	struct cgroup *cgrp;
> > +
> > +	spin_lock_irq(&css_set_lock);
> > +	cgrp = task_dfl_cgroup(current);
> > +	if (always_leave || !test_bit(CGRP_FREEZE, &cgrp->flags)) {
> > +		cgroup_dec_frozen_cnt(cgrp);
> > +		cgroup_update_frozen(cgrp);
> > +		WARN_ON_ONCE(!current->frozen);
> > +		current->frozen = false;
> > +	}
> > +	spin_unlock_irq(&css_set_lock);
> > +
> > +	if (unlikely(current->frozen)) {
> > +		/*
> > +		 * If the task remained in the frozen state,
> > +		 * make sure it won't reach userspace without
> > +		 * entering the signal handling loop.
> > +		 */
> > +		spin_lock_irq(&current->sighand->siglock);
> > +		recalc_sigpending();
> > +		spin_unlock_irq(&current->sighand->siglock);
>
> I still can't understand this logic.
>
> Once again, suppose we race with CGRP_FREEZE. If JOBCTL_TRAP_FREEZE is already
> set then signal_pending() must be already T and we do not need recalc_sigpending?
> If JOBCTL_TRAP_FREEZE is not set yet, how can recalc_sigpending() help?
>
> > +static void cgroup_freeze_task(struct task_struct *task, bool freeze)
> > +{
> > +	unsigned long flags;
> > +
> > +	/* If the task is about to die, don't bother with freezing it. */
> > +	if (!lock_task_sighand(task, &flags))
> > +		return;
> > +
> > +	if (freeze) {
> > +		task->jobctl |= JOBCTL_TRAP_FREEZE;
> > +		signal_wake_up(task, false);
> > +	} else {
> > +		task->jobctl &= ~JOBCTL_TRAP_FREEZE;
> > +		wake_up_process(task);
>
> wake_up_interruptible() ?
>
> >  static int ptrace_signal(int signr, kernel_siginfo_t *info)
> >  {
> >  	/*
> > @@ -2442,6 +2483,10 @@ bool get_signal(struct ksignal *ksig)
> >  		ksig->info.si_signo = signr = SIGKILL;
> >  		sigdelset(&current->pending.signal, SIGKILL);
> >  		recalc_sigpending();
> > +		current->jobctl &= ~JOBCTL_TRAP_FREEZE;
> > +		spin_unlock_irq(&sighand->siglock);
> > +		if (unlikely(cgroup_task_frozen(current)))
> > +			cgroup_leave_frozen(true);
>
> Oh, and another leave_frozen below...
>
> I feel this must be simplified somehow, but nothing comes to my mind right now.
>
> > +		/*
> > +		 * If the task is leaving the frozen state, let's update
> > +		 * cgroup counters and reset the frozen bit.
> > +		 */
> > +		if (unlikely(cgroup_task_frozen(current))) {
> >  			spin_unlock_irq(&sighand->siglock);
> > +			cgroup_leave_frozen(true);
> >  			goto relock;
> >  		}
>
> afaics cgroup_leave_frozen(false) makes more sense here.
>
> Oleg.