[PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[qiwuchen55]

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);
-	struct private_data *priv = policy->driver_data;
+	struct cpufreq_policy policy;
+	struct private_data *priv;
+
+	/*
+	 * In case cpufreq policy has been released, just return 0.
+	 */
+	if (cpufreq_get_policy(&policy, cpu))
+		return 0;
+
+	priv = policy.driver_data;
 
 	return brcm_avs_get_frequency(priv->base);
 }
-- 
1.9.1


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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[Viresh Kumar]

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 ?

> -	struct private_data *priv = policy->driver_data;
> +	struct cpufreq_policy policy;
> +	struct private_data *priv;
> +
> +	/*
> +	 * In case cpufreq policy has been released, just return 0.
> +	 */
> +	if (cpufreq_get_policy(&policy, cpu))
> +		return 0;

Why did you move away from the previous implementation of cpufreq_cpu_get() ?

> +
> +	priv = policy.driver_data;
>  
>  	return brcm_avs_get_frequency(priv->base);
>  }
> -- 
> 1.9.1

-- 
viresh

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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[chenqiwu]

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?

> > -	struct private_data *priv = policy->driver_data;
> > +	struct cpufreq_policy policy;
> > +	struct private_data *priv;
> > +
> > +	/*
> > +	 * In case cpufreq policy has been released, just return 0.
> > +	 */
> > +	if (cpufreq_get_policy(&policy, cpu))
> > +		return 0;
> 
> Why did you move away from the previous implementation of cpufreq_cpu_get() ?
> 
> > +
> > +	priv = policy.driver_data;
> >  
> >  	return brcm_avs_get_frequency(priv->base);
> >  }
> > -- 
> > 1.9.1
> 
> -- 
> viresh

Qiwu

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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[Viresh Kumar]

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

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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[chenqiwu]

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?
If the policy has been released, cpufreq_get_policy() will return -EINVAL,
it won't do an extra memcpy.

Qiwu

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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[Viresh Kumar]

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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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[chenqiwu]

On Mon, Jan 20, 2020 at 11:51:26AM +0530, Viresh Kumar wrote:
> 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.
>
Is there any question about this extra memcpy?

Qiwu

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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[Viresh Kumar]

On 20-01-20, 14:27, chenqiwu wrote:
> On Mon, Jan 20, 2020 at 11:51:26AM +0530, Viresh Kumar wrote:
> > 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.
> >
> Is there any question about this extra memcpy?

What do you mean by that?

The whole point I am trying to make is that for your specific case, doing an
explicit cpufreq_cpu_get() and cpufreq_cpu_put() is far more efficient than
calling cpufreq_get_policy() which has a different purpose and usecase.

-- 
viresh

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Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount

[chenqiwu]

On Mon, Jan 20, 2020 at 12:00:04PM +0530, Viresh Kumar wrote:
> On 20-01-20, 14:27, chenqiwu wrote:
> > On Mon, Jan 20, 2020 at 11:51:26AM +0530, Viresh Kumar wrote:
> > > 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.
> > >
> > Is there any question about this extra memcpy?
> 
> What do you mean by that?
> 
> The whole point I am trying to make is that for your specific case, doing an
> explicit cpufreq_cpu_get() and cpufreq_cpu_put() is far more efficient than
> calling cpufreq_get_policy() which has a different purpose and usecase.
>

For efficiency, I agree your idea.
So we have change as follows:
diff --git a/drivers/cpufreq/brcmstb-avs-cpufreq.c b/drivers/cpufreq/brcmstb-avs-cpufreq.c
index 77b0e5d..b2ddde3 100644
--- a/drivers/cpufreq/brcmstb-avs-cpufreq.c
+++ b/drivers/cpufreq/brcmstb-avs-cpufreq.c
@@ -455,6 +455,11 @@ static unsigned int brcm_avs_cpufreq_get(unsigned int cpu)
        struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
        struct private_data *priv = policy->driver_data;

+       if (!policy)
+               return 0;
+
+       cpufreq_cpu_put(policy);
+
        return brcm_avs_get_frequency(priv->base);
 }

Qiwu

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