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

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Date:	Thu, 3 Nov 2011 00:11:23 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...hat.com>
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 Wednesday, November 02, 2011, Oleg Nesterov wrote:
> Hi,
> 
> On 11/01, Tejun Heo wrote:
> >
> > For now, let's go with the count/dont_count.  Can you please write up
> > a patch for that?  Jeff, does this seem okay to you?
> 
> OK, will do in a minute. On top of
> "[PATCH pm] freezer: fix wait_event_freezable/__thaw_task races"
> you sent. (btw, thanks, I forgout about it ;)
> 
> Rafael, could you remind why freezer_do_not_count/freezer_count check
> ->mm != NULL ?

You're asking difficult questions. ;-)

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.

> 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).

Thanks,
Rafael
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