linux-kernel - Re: [PATCH] cpuidle: Check cpuidle driver before add refcount

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Message-ID: <1725801.dn2z0gQifB@vostro.rjw.lan>
Date:	Fri, 30 Aug 2013 15:25:32 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Daniel Fu <danifu@...dia.com>
Cc:	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Johnny Qiu <joqiu@...dia.com>
Subject: Re: [PATCH] cpuidle: Check cpuidle driver before add refcount

On Friday, August 30, 2013 08:26:34 PM Daniel Fu wrote:
> X-NVConfidentiality: public
> 
> In SMP, When doing the cpuidle driver registration. There maybe only 1 cpu have register cpuidle(CPU0), 
> but In the process of registration cpuidle driver, the scheduler maybe schedule the process to the other CPU (eg. CPU0 have heay load, migrate to CPU1) 
> cpuidle_get_driver() --> get_cpu() , will get CPU1, if CPU1 didn't register cpuidle driver, Will get NULL.
> 
> I know we should prevent the registration migrate to the other CPU, but we should check drv before add refcount to prevent kernel panic at least, Right?

Well, cpuidle_driver_ref() has only one caller, which is not the registration.

However, the caller apparently assumes that that function may return NULL and
cpuidle_driver_unref() check drv against NULL too, so cpuidle_driver_ref()
should do that either.

Did you see that problem in testing or just through code inspection?

Rafael


> -----Original Message-----
> From: Rafael J. Wysocki [mailto:rjw@...k.pl] 
> Sent: 2013年8月30日 20:01
> To: Daniel Fu
> Cc: daniel.lezcano@...aro.org; linux-pm@...r.kernel.org; linux-kernel@...r.kernel.org; Johnny Qiu
> Subject: Re: [PATCH] cpuidle: Check cpuidle driver before add refcount
> 
> On Friday, August 30, 2013 07:48:22 PM Daniel Fu wrote:
> > If the current CPU has no cpuidle driver, drv will be NULL.
> > Check if we get drv successfully before add refount to prevent Kernel 
> > panic.
> 
> What is the actual scenario that may lead to this panic?
> 
> Rafael
> 
> 
> > Signed-off-by: Daniel Fu <danifu@...dia.com>
> > ---
> >  drivers/cpuidle/driver.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c index 
> > 3ac499d..6e11701 100644
> > --- a/drivers/cpuidle/driver.c
> > +++ b/drivers/cpuidle/driver.c
> > @@ -331,7 +331,8 @@ struct cpuidle_driver *cpuidle_driver_ref(void)
> >  	spin_lock(&cpuidle_driver_lock);
> >  
> >  	drv = cpuidle_get_driver();
> > -	drv->refcnt++;
> > +	if (drv)
> > +		drv->refcnt++;
> >  
> >  	spin_unlock(&cpuidle_driver_lock);
> >  	return drv;
> > 
> --
> I speak only for myself.
> Rafael J. Wysocki, Intel Open Source Technology Center.
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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