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Subject: Re: [PATCH 4.4 092/134] cpufreq: Fix governor module removal race
From: Ben Hutchings <ben.hutchings@codethink.co.uk>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Sasha Levin <alexander.levin@microsoft.com>
Date: Sun, 01 Apr 2018 21:56:41 +0100
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         <20180319171902.578082436@linuxfoundation.org>
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On Mon, 2018-03-19 at 19:06 +0100, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
> 
> 
> [ Upstream commit a8b149d32b663c1a4105273295184b78f53d33cf ]
[...]
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -551,6 +551,8 @@ static int cpufreq_parse_governor(char *
>  			*governor = t;
>  			err = 0;
>  		}
> +		if (t && !try_module_get(t->owner))
> +			t = NULL;

This won't work because t is dead after this point.  The fix appears to
depend on:

commit 045149e6a22119e5bf0d16a0b24a4173a2abb71d
Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Date:   Thu Nov 23 01:23:16 2017 +0100

    cpufreq: Clean up cpufreq_parse_governor()

which moves the assignment to *governor further down.

Ben.
 
>  		mutex_unlock(&cpufreq_governor_mutex);
>  	}
> @@ -669,6 +671,10 @@ static ssize_t store_scaling_governor(st
>  		return -EINVAL;
>  
>  	ret = cpufreq_set_policy(policy, &new_policy);
> +
> +	if (new_policy.governor)
> +		module_put(new_policy.governor->owner);
> +
>  	return ret ? ret : count;
>  }
>  

-- 
Ben Hutchings
Software Developer, Codethink Ltd.