On Mon, Jan 20, 2020 at 12:00:04PM +0530, Viresh Kumar wrote:

On 20-01-20, 14:27, chenqiwu wrote:

quoted

On Mon, Jan 20, 2020 at 11:51:26AM +0530, Viresh Kumar wrote:

quoted

On 20-01-20, 14:13, chenqiwu wrote:

quoted

On Mon, Jan 20, 2020 at 11:31:34AM +0530, Viresh Kumar wrote:

quoted

On 20-01-20, 13:58, chenqiwu wrote:

quoted

On Mon, Jan 20, 2020 at 11:02:50AM +0530, Viresh Kumar wrote:

quoted

On 19-01-20, 15:09, qiwuchen55@gmail.com wrote:

quoted

From: chenqiwu <redacted>

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 <redacted>
---
 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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