linux-kernel - Re: [RFC PATCH] cpufreq/hotplug: Fix cpu-hotplug cpufreq race conditions

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Date:	Tue, 2 Jun 2015 11:41:33 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	rjw@...ysocki.net, ego@...ux.vnet.ibm.com, paulus@...ba.org,
	linux-kernel@...r.kernel.org, shilpa.bhat@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH] cpufreq/hotplug: Fix cpu-hotplug cpufreq race
 conditions

On 02-06-15, 11:33, Preeti U Murthy wrote:
> No, dbs_data is a governor wide data structure and not a policy wide

Yeah, that's the common part which I was referring to. But normally
its just read for policies in START/STOP, they just update per-cpu
data for policy->cpus.

> one, which is manipulated in START/STOP calls for drivers where the
> CPUFREQ_HAVE_GOVERNOR_PER_POLICY is not set.
> 
> So even if we assume that we hold per-policy locks, the following race
> is still present. Assume that we have just two cpus which do not have a
> governor-per-policy set.
> 
> CPU0                        CPU1
> 
>  store*                     store*
> 
> lock(policy 1)              lock(policy 2)
> cpufreq_set_policy()       cpufreq_set_policy()
> EXIT() :
> dbs-data->usage_count--
> 
> INIT()
> dbs_data exists

You missed the usage_count++ here.

> so return
>                             EXIT()
>                             dbs_data->usage_count -- = 0
>                             kfree(dbs_data)

                                And so this shouldn't happen. Else we
                                are missing locking in governor's
                                code, rather than cpufreq.c

-- 
viresh
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