Re: [PATCH 02/10] cpufreq: conservative: Avoid races with transition notifier

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 06/22/2015 01:32 PM, Viresh Kumar wrote:
> It is possible that cpufreq transition notifier is called while the
> governor is performing its EXIT operation. If this happens, 'ccdbs'

When does this happen ? As far as I can see, cpufreq transition notifier
gets called from the cpufreq kworker or when we set the cpufreq limits.
And from your previous patches, an exit operation only proceeds after
ensuring that no kworker is running (check on ccdbs->policy). And LIMIT
operation does not run in parallel too.

Regards
Preeti U Murthy

> may get updated to NULL, while it is being accessed from the notifier
> callback. And that will result in NULL pointer dereference.
> 
> ccdbs is used here just to get cpufreq policy, which can be obtained
> from cpufreq_cpu_get() as well. And so the reference to ccdbs can be
> avoided.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  drivers/cpufreq/cpufreq_conservative.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq_conservative.c b/drivers/cpufreq/cpufreq_conservative.c
> index 0e4154e584bf..1e3cabfb2b57 100644
> --- a/drivers/cpufreq/cpufreq_conservative.c
> +++ b/drivers/cpufreq/cpufreq_conservative.c
> @@ -119,12 +119,13 @@ static int dbs_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
>  	struct cpufreq_freqs *freq = data;
>  	struct cs_cpu_dbs_info_s *dbs_info =
>  					&per_cpu(cs_cpu_dbs_info, freq->cpu);
> -	struct cpufreq_policy *policy;
> +	struct cpufreq_policy *policy = cpufreq_cpu_get(freq->cpu);
> 
> -	if (!dbs_info->enable)
> +	if (!policy)
>  		return 0;
> 
> -	policy = dbs_info->cdbs.ccdbs->policy;
> +	if (!dbs_info->enable)
> +		goto policy_put;
> 
>  	/*
>  	 * we only care if our internally tracked freq moves outside the 'valid'
> @@ -134,6 +135,9 @@ static int dbs_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
>  			|| dbs_info->requested_freq < policy->min)
>  		dbs_info->requested_freq = freq->new;
> 
> +policy_put:
> +	cpufreq_cpu_put(policy);
> +
>  	return 0;
>  }
>