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SubjectRe: [PATCH 3/4] cpufreq: Fix governor module removal race
Date
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@intel.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@intel.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

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