linux-kernel - Re: od_set_powersave_bias: NULL pointer dereference

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Message-ID: <20130626175722.GA20226@jshin-Toonie>
Date:	Wed, 26 Jun 2013 12:57:22 -0500
From:	Jacob Shin <jacob.shin@....com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Tim Gardner <tim.gardner@...onical.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>, <cpufreq@...r.kernel.org>,
	<linux-pm@...r.kernel.org>
Subject: Re: od_set_powersave_bias: NULL pointer dereference

On Wed, Jun 26, 2013 at 08:02:29PM +0530, Viresh Kumar wrote:
> On 26 June 2013 19:58, Jacob Shin <jacob.shin@....com> wrote:
> > On Wed, Jun 26, 2013 at 12:18:27PM +0530, Viresh Kumar wrote:
> 
> >> I am not sure if this is enough. What if we had ondemand as the
> >> governor initially, then we changed it to something else. Now also
> >> cur_policy contains a address and isn't zero.

I just tested this case with this patch applied, and did not have any
problems.

> >>
> >> >                 cpumask_or(&done, &done, policy->cpus);
> >> > +
> >> > +               if (policy->governor != &cpufreq_gov_ondemand)
> >> > +                       continue;
> >
> > This should catch that case no ?
> 
> Policy might be freed and reallocated by then. And so doing
> policy->governor is dangerous.

Are you worried that after we have passed the above if check, and
before we access ->tuner governor change might occur?

Is there something synonymous to get/put_online_cpus() for cpufreq to
prevent governor change while we update ->tuner values?

Otherwise, should just spinlock?

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