linux-kernel - Re: [PATCH 3/4] cpufreq: Fix governor module removal race

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Message-ID: <1684964.YkVv9kPgaH@aspire.rjw.lan>
Date:   Thu, 23 Nov 2017 14:16:08 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH 3/4] cpufreq: Fix governor module removal race

On Thursday, November 23, 2017 5:01:17 AM CET Viresh Kumar wrote:
> On 23-11-17, 01:29, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > 
> > It is possible to remove a cpufreq governor module after
> > cpufreq_parse_governor() has returned success in
> > store_scaling_governor() and before cpufreq_set_policy()
> > acquires a reference to it, because the governor list is
> > not protected during that period and nothing prevents the
> > governor from being unregistered then.  The pointer to the
> > governor structure coming from cpufreq_parse_governor() may
> > become stale as a result of that.
> > 
> > Prevent that from happening by acquiring an extra reference
> > to the governor module temporarily in cpufreq_parse_governor(),
> > under cpufreq_governor_mutex, and dropping it in
> > store_scaling_governor(), when cpufreq_set_policy() returns.
> > 
> > Note that the second cpufreq_parse_governor() call site is fine,
> > because it only cares about the policy member of new_policy.
> > 
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/cpufreq/cpufreq.c |    8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > Index: linux-pm/drivers/cpufreq/cpufreq.c
> > ===================================================================
> > --- linux-pm.orig/drivers/cpufreq/cpufreq.c
> > +++ linux-pm/drivers/cpufreq/cpufreq.c
> > @@ -607,11 +607,13 @@ static int cpufreq_parse_governor(char *
> >  	if (cpufreq_driver->setpolicy) {
> >  		if (!strncasecmp(str_governor, "performance", CPUFREQ_NAME_LEN)) {
> >  			policy->policy = CPUFREQ_POLICY_PERFORMANCE;
> > +			policy->governor = NULL;
> >  			return 0;
> >  		}
> >  
> >  		if (!strncasecmp(str_governor, "powersave", CPUFREQ_NAME_LEN)) {
> >  			policy->policy = CPUFREQ_POLICY_POWERSAVE;
> > +			policy->governor = NULL;
> 
> Why are the above two changes required? policy->governor should always be NULL
> for setpolicy drivers anyway.

OK, I'll drop them.

Thanks,
Rafael