Messages in this thread | | | Date | Tue, 02 Jun 2015 11:33:52 +0530 | From | Preeti U Murthy <> | Subject | Re: [RFC PATCH] cpufreq/hotplug: Fix cpu-hotplug cpufreq race conditions |
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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
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