linux-kernel - Re: [PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks (rev. 2)

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Date:	Tue, 3 Jul 2007 15:36:58 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Pavel Machek <pavel@....cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Uli Luckas <u.luckas@...d.de>, Oleg Nesterov <oleg@...sign.ru>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks (rev. 2)

On Tuesday, 3 July 2007 15:19, Rafael J. Wysocki wrote:
> On Tuesday, 3 July 2007 07:30, Benjamin Herrenschmidt wrote:
> > On Tue, 2007-06-26 at 00:27 +0200, Rafael J. Wysocki wrote:
> > > > >     case PM_HIBERNATION_PREPARE:
> > > > >     case PM_SUSPEND_PREPARE:
> > > > >             usermodehelper_disabled = 1;
> > > > > -           return NOTIFY_OK;
> > > > > +           smp_mb();
> > > > 
> > > > usermodehelper_disabled should be atomic variable, too, so we don't
> > > > have to play these ugly tricks by hand? This should not be
> > > > performance-critical, right?
> > > 
> > > Well, I think we'd need to add the barriers anyway.
> > > 
> > > The problem, as far as I understand it, is that the instructions can
> > > get
> > > reordered if there are no barriers in there.
> > 
> > That seems dodgy either way to me :-)
> > 
> > Just use a spinlock.
> 
> Around wait_for_completion()?  I don't think that's a good idea. :-)

Sorry, I mistunderstood you (I think).

Yes, I could use a spinlock for protecting usermodehelper_disabled, but
why would that be better than the current code?

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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