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SubjectRe: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount
On 20-01-20, 14:13, chenqiwu wrote:
> On Mon, Jan 20, 2020 at 11:31:34AM +0530, Viresh Kumar wrote:
> > On 20-01-20, 13:58, chenqiwu wrote:
> > > On Mon, Jan 20, 2020 at 11:02:50AM +0530, Viresh Kumar wrote:
> > > > On 19-01-20, 15:09, qiwuchen55@gmail.com wrote:
> > > > > From: chenqiwu <chenqiwu@xiaomi.com>
> > > > >
> > > > > brcm_avs_cpufreq_get() calls cpufreq_cpu_get() to get the cpufreq policy,
> > > > > meanwhile, it also increments the kobject reference count to mark it busy.
> > > > > However, a corresponding call of cpufreq_cpu_put() is ignored to decrement
> > > > > the kobject reference count back, which may lead to a potential stuck risk
> > > > > that the cpuhp thread deadly waits for dropping of kobject refcount when
> > > > > cpufreq policy free.
> > > > >
> > > > > For fixing this bug, cpufreq_get_policy() is referenced to do a proper
> > > > > cpufreq_cpu_get()/cpufreq_cpu_put() and fill a policy copy for the user.
> > > > > If the policy return NULL, we just return 0 to hit the code path of
> > > > > cpufreq_driver->get.
> > > > >
> > > > > Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
> > > > > ---
> > > > > drivers/cpufreq/brcmstb-avs-cpufreq.c | 12 ++++++++++--
> > > > > 1 file changed, 10 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/cpufreq/brcmstb-avs-cpufreq.c b/drivers/cpufreq/brcmstb-avs-cpufreq.c
> > > > > index 77b0e5d..ee0d404 100644
> > > > > --- a/drivers/cpufreq/brcmstb-avs-cpufreq.c
> > > > > +++ b/drivers/cpufreq/brcmstb-avs-cpufreq.c
> > > > > @@ -452,8 +452,16 @@ static bool brcm_avs_is_firmware_loaded(struct private_data *priv)
> > > > >
> > > > > static unsigned int brcm_avs_cpufreq_get(unsigned int cpu)
> > > > > {
> > > > > - struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> > > >
> > > > Why can't we just add a corresponding cpufreq_cpu_put() instead of all this ?
> > > >
> > >
> > > cpufreq_get_policy() does a proper cpufreq_cpu_get()/cpufreq_cpu_put(),
> > > meanwhile fills a policy copy for the user. It equals to using
> > > cpufreq_cpu_get() and a corresponding cpufreq_cpu_put() around access
> > > to the policy pointer. I think both methods are fine here.
> > > What do you think?
> >
> > cpufreq_get_policy() does an extra memcpy as well, which isn't required at all
> > in your case.
> >
> > --
> > viresh
>
> Huha..Do you worry about the race conditon with cpufreq policy free path?

No. I just worry about an unnecessary memcpy, nothing else.

--
viresh

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