linux-kernel - Re: [PATCH pm-freezer 1/4] cgroup_freezer: fix freezer->state setting bug in freezer_change

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Date:	Fri, 2 Sep 2011 19:15:17 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Paul Menage <paul@...lmenage.org>,
	containers@...ts.linux-foundation.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH pm-freezer 1/4] cgroup_freezer: fix freezer->state
	setting bug in freezer_change_state()

On 09/03, Tejun Heo wrote:
>
> Hello,
>
> On Fri, Sep 02, 2011 at 06:58:39PM +0200, Oleg Nesterov wrote:
> > > This still doesn't look quite right. If the cgroup is FREEZING it should
> > > also call try_to_freeze_cgroup(). I think this is what's needed:
> > >
> > > 		if (freezer->state == CGROUP_THAWED)
> > > 			atomic_inc(&system_freezing_cnt);
> > > 		freezer->state = CGROUP_FREEZING;
> > > 		retval = try_to_freeze_cgroup(cgroup, freezer);
> >
> > This is what I mentioned before, to me this looks like a win.
> >
> > Why do we need try_to_freeze_cgroup() in this case? "for safety"
> > could actually mean "hide the bug" ;)
>
> I guess it depends on the viewpoint.  A simple analogy would be using
> WARN_ON_ONCE() instead of BUG_ON() so that the mode of failure is
> softer.  This change isn't likely to make bugs significantly more
> difficult to discover so why not?

I agree either way.

Personally I prefer your current patch. Because it is not clear why
do we call try_to_freeze_cgroup() if it was already called. And, the
2nd call can silently hide the problem if we have some bug.

But of course, this is up to you and Matt.



> > But I agree either way. Rafael, I think 1-4 are fine, but I think
> > we need the simple 5/4, will send in a minute...
>
> Can you please wait a bit?  The second one was broken (missing unlock)

Yes, I just noticed the small problem too, hopefully we mean the same
bug ;)

Oleg.

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