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, 26 Jun 2007 16:02:41 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Pavel Machek <pavel@....cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Uli Luckas <u.luckas@...d.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks (rev. 2)

On 06/26, Rafael J. Wysocki wrote:
>
> On Monday, 25 June 2007 23:55, Pavel Machek 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.

Yes, and it doesn't help if we make usermodehelper_disabled atomic_t.
atomic_xxx() operations do not imply the memory barrier semantics.

Oleg.

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