linux-kernel - Re: [RFC PATCH] freezer: revert 27920651fe "PM / Freezer: Make fake_signal_wake_up() wake TASK

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Message-ID: <20111103151033.GA26843@redhat.com>
Date:	Thu, 3 Nov 2011 16:10:33 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Tejun Heo <tj@...nel.org>, Jeff Layton <jlayton@...hat.com>,
	Steve French <sfrench@...ba.org>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-cifs@...r.kernel.org,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>, trond.myklebust@...app.com,
	linux-nfs@...r.kernel.org
Subject: Re: [RFC PATCH] freezer: revert 27920651fe "PM / Freezer: Make
	fake_signal_wake_up() wake TASK_KILLABLE tasks too"

On 11/03, Rafael J. Wysocki wrote:
>
> On Wednesday, November 02, 2011, Oleg Nesterov wrote:
> >
> > Rafael, could you remind why freezer_do_not_count/freezer_count check
> > ->mm != NULL ?
>
> You're asking difficult questions. ;-)

I am trying ;)

> The intention was to prevent PF_FREEZER_SKIP from having any effect on
> kernel threads, IIRC.  Anyway, there are only two legitimate users of it
> (vfork and apm_ioctl) and in both cases the task in question is user space.

Actually CLONE_VFORK is used by call_usermodehelper() paths but this case
is fine. The caller is the PF_NOFREEZE workqueue thread.

> > The comment says "However, we don't want kernel threads to be frozen",
> > but it is not clear anyway. A kernel thread simply shouldn't use this
> > interface if it doesn't want to freeze.
> >
> > And in any case, PF_KTHREAD looks better if we really need to filter
> > out the kernel threads.
>
> PF_FREEZER_SKIP was introduced specifically with vfork in mind and I'm not
> sure if it's a good idea to re-use it for something else (at least not for
> something entirely obvious).

Indeed! So why do we check ->mm != NULL?

We can remove this check, right now it doesn't matter. And we are trying
to avoid the new users of this interface.

Oleg.

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