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

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Message-ID: <20070703174503.GB108@tv-sign.ru>
Date:	Tue, 3 Jul 2007 21:45:03 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, 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 07/03, 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.

Actually, spinlock_t is not suitable. Because spin_unlcok() does NOT imply
mb(). The subsequent wait_event_timeout()->atomic_read() may leak into the
critical section.

We can use set_mb(), if we don't want to play with smp_mb() by hand :)

Oleg.

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