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

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Message-ID: <20200120055822.GB5185@cqw-OptiPlex-7050>
Date:   Mon, 20 Jan 2020 13:58:22 +0800
From:   chenqiwu <qiwuchen55@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     mmayer@...adcom.com, rjw@...ysocki.net, f.fainelli@...il.com,
        bcm-kernel-feedback-list@...adcom.com, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        chenqiwu <chenqiwu@...omi.com>
Subject: Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy
 refcount

On Mon, Jan 20, 2020 at 11:02:50AM +0530, Viresh Kumar wrote:
> On 19-01-20, 15:09, qiwuchen55@...il.com wrote:
> > From: chenqiwu <chenqiwu@...omi.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@...omi.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