From: Preeti U Murthy <preeti@linux.vnet.ibm.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: rjw@rjwysocki.net, ego@linux.vnet.ibm.com, paulus@samba.org,
linux-kernel@vger.kernel.org, shilpa.bhat@linux.vnet.ibm.com,
linux-pm@vger.kernel.org
Subject: Re: [RFC PATCH] cpufreq/hotplug: Fix cpu-hotplug cpufreq race conditions
Date: Tue, 02 Jun 2015 11:33:52 +0530 [thread overview]
Message-ID: <556D4748.7040105@linux.vnet.ibm.com> (raw)
In-Reply-To: <20150602053956.GD10443@linux>
On 06/02/2015 11:09 AM, Viresh Kumar wrote:
> On 02-06-15, 11:01, Preeti U Murthy wrote:
>> How will a policy lock help here at all, when cpus from multiple
>> policies are calling into __cpufreq_governor() ? How will a policy lock
>> serialize their entry into cpufreq_governor_dbs() ?
>
> So different policies don't really depend on each other. The only
> thing common to them are the governor's sysfs files (only if
> governor-per-policy isn't set, i.e. in your case). Those sysfs files
> and their kernel counterpart variables aren't touched unless all the
> policies have EXITED. All these START/STOP calls touch only the data
> relevant to those policies only.
No, dbs_data is a governor wide data structure and not a policy wide
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
so return
EXIT()
dbs_data->usage_count -- = 0
kfree(dbs_data)
START()
dereference dbs_data
*NULL dereference*
Regards
Preeti U Murthy
next prev parent reply other threads:[~2015-06-02 6:04 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-01 6:40 [RFC PATCH] cpufreq/hotplug: Fix cpu-hotplug cpufreq race conditions Preeti U Murthy
2015-06-01 7:19 ` Viresh Kumar
2015-06-01 7:55 ` Preeti U Murthy
2015-06-02 5:31 ` Preeti U Murthy
2015-06-02 5:39 ` Viresh Kumar
2015-06-02 6:03 ` Preeti U Murthy [this message]
2015-06-02 6:11 ` Viresh Kumar
2015-06-02 6:20 ` Preeti U Murthy
2015-06-02 6:27 ` Viresh Kumar
2015-06-02 6:56 ` Preeti U Murthy
2015-06-02 7:07 ` Viresh Kumar
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