linux-kernel - Re: [PATCH V2 1/2] cpufreq: unlock correct rwsem while updating policy->cpu

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Date:	Tue, 17 Sep 2013 10:24:46 +0100
From:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	rjw@...k.pl, linaro-kernel@...ts.linaro.org, patches@...aro.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, srivatsa.bhat@...ux.vnet.ibm.com
Subject: Re: [PATCH V2 1/2] cpufreq: unlock correct rwsem while updating
 policy->cpu

On Tue, 2013-09-17 at 10:22 +0530, Viresh Kumar wrote:
> Current code looks like this:
> 
>         WARN_ON(lock_policy_rwsem_write(cpu));
>         update_policy_cpu(policy, new_cpu);
>         unlock_policy_rwsem_write(cpu);
> 
> {lock|unlock}_policy_rwsem_write(cpu) takes/releases policy->cpu's rwsem.
> Because cpu is changing with the call to update_policy_cpu(), the
> unlock_policy_rwsem_write() will release the incorrect lock.
> 
> The right solution would be to release the same lock as was taken earlier. Also
> update_policy_cpu() was also called from cpufreq_add_dev() without any locks and
> so its better if we move this locking to inside update_policy_cpu().
> 
> Reported-and-Tested-by: Jon Medhurst<tixy@...aro.org>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> Hi Rafael,
> 
> Only one patch is sent now as other one is unchanged.

This patch fixes a regression introduced in 3.12 by commit f9ba680d23
(cpufreq: Extract the handover of policy cpu to a helper function).

The other patch is a tidyup of long-standing code.

>  drivers/cpufreq/cpufreq.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 43c24aa..1479522 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -952,9 +952,20 @@ static void update_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)
>  	if (cpu == policy->cpu)
>  		return;
>  
> +	/*
> +	 * Take direct locks as lock_policy_rwsem_write wouldn't work here.
> +	 * Also lock for last cpu is enough here as contention will happen only
> +	 * after policy->cpu is changed and after it is changed, other threads
> +	 * will try to acquire lock for new cpu. And policy is already updated
> +	 * by then.
> +	 */
> +	down_write(&per_cpu(cpu_policy_rwsem, policy->cpu));
> +
>  	policy->last_cpu = policy->cpu;
>  	policy->cpu = cpu;
>  
> +	up_write(&per_cpu(cpu_policy_rwsem, policy->last_cpu));
> +
>  #ifdef CONFIG_CPU_FREQ_TABLE
>  	cpufreq_frequency_table_update_policy_cpu(policy);
>  #endif
> @@ -1203,9 +1214,7 @@ static int __cpufreq_remove_dev_prepare(struct device *dev,
>  
>  		new_cpu = cpufreq_nominate_new_policy_cpu(policy, cpu, frozen);
>  		if (new_cpu >= 0) {
> -			WARN_ON(lock_policy_rwsem_write(cpu));
>  			update_policy_cpu(policy, new_cpu);
> -			unlock_policy_rwsem_write(cpu);
>  
>  			if (!frozen) {
>  				pr_debug("%s: policy Kobject moved to cpu: %d "

-- 
Tixy

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