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

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Message-Id: <200706251703.42979.rjw@sisk.pl>
Date:	Mon, 25 Jun 2007 17:03:42 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Pavel Machek <pavel@....cz>, 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

On Monday, 25 June 2007 12:43, Oleg Nesterov wrote:
> Rafael J. Wysocki wrote:
> >
> >  static int usermodehelper_pm_callback(struct notifier_block *nfb,
> >  					unsigned long action,
> >  					void *ignored)
> >  {
> > +	long retval;
> > +
> >  	switch (action) {
> >  	case PM_HIBERNATION_PREPARE:
> >  	case PM_SUSPEND_PREPARE:
> >  		usermodehelper_disabled = 1;
> > -		return NOTIFY_OK;
> > +		/*
> > +		 * From now on call_usermodehelper_exec() won't start any new
> > +		 * helpers, so it is sufficient if running_helpers turns out to
> > +		 * be zero at one point (it may be increased later, but that
> > +		 * doesn't matter).
> > +		 */
> > +		retval = wait_event_timeout(running_helpers_waitq,
> > +					atomic_read(&running_helpers) == 0,
> > +					RUNNING_HELPERS_TIMEOUT);
> > +		if (retval) {
> > +			return NOTIFY_OK;
> > +		} else {
> > +			usermodehelper_disabled = 0;
> > +			return NOTIFY_BAD;
> 
> I think this is racy. First, this needs smp_mb() between "usermodehelper_disabled = 1"
> and wait_event_timeout().

Yes ...

> Second, call_usermodehelper's path should first increment the counter, and only
> then check usermodehelper_disabled,

It does this already.

> and it needs an mb() in between too. Otherwise, the helper can see
> usermodehelper_disabled == 0, then PM_SUSPEND_PREPARE comes and 
> returns NOTIFY_OK, then the helper increments the counter and starts application.
> 
> Sadly, you can't use srcu/qrcu because it doesn't handle timeouts.	

OK

If I understand that correctly, it should suffice to place smp_mb() after
usermodehelper_disabled = 1; in usermodehelper_pm_callback() and another
smp_mb() after atomic_inc(&running_helpers); in new_helper().

Is that correct?

Greetings,
Rafael


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