linux-kernel - Re: [PATCH v8 4/7] cgroup: cgroup v2 freezer

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Date:   Thu, 21 Feb 2019 17:44:04 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Roman Gushchin <guro@...com>
Cc:     Roman Gushchin <guroan@...il.com>, Tejun Heo <tj@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 4/7] cgroup: cgroup v2 freezer

On 02/20, Roman Gushchin wrote:
>
> On Wed, Feb 20, 2019 at 03:42:02PM +0100, Oleg Nesterov wrote:
> > On 02/19, Roman Gushchin wrote:
> > >
> > > @@ -2363,7 +2419,8 @@ bool get_signal(struct ksignal *ksig)
> > >  	 * we should notify the parent, prepare_signal(SIGCONT) encodes
> > >  	 * the CLD_ si_code into SIGNAL_CLD_MASK bits.
> > >  	 */
> > > -	if (unlikely(signal->flags & SIGNAL_CLD_MASK)) {
> > > +	if (unlikely(signal->flags & SIGNAL_CLD_MASK) &
> > > +	    !(current->jobctl & JOBCTL_TRAP_FREEZE)) {
> >
> > Hmm, why?
>
> If a process is going from the stopped to the frozen state, it looks like
> it's better to delay parent's notification up to the moment when it will
> actually run.

Perhaps I missed something but I do not understand why it is really better,
at least why it deserves this (small but still) complication...

OK, if nothing else, suppose that SIGCONT races with cgroup_do_freeze(true).
With or without this change CLD_CONTINUED can be reported or not depending
on /dev/random, do we really care?

Oleg.