From mboxrd@z Thu Jan 1 00:00:00 1970 From: Preeti U Murthy Subject: Re: [RFC PATCH] cpufreq/hotplug: Fix cpu-hotplug cpufreq race conditions Date: Tue, 02 Jun 2015 11:33:52 +0530 Message-ID: <556D4748.7040105@linux.vnet.ibm.com> References: <20150601064031.2972.59208.stgit@perfhull-ltc.austin.ibm.com> <20150601071934.GC4242@linux> <556D3FAA.3080703@linux.vnet.ibm.com> <20150602053956.GD10443@linux> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e33.co.us.ibm.com ([32.97.110.151]:40630 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752585AbbFBGD7 (ORCPT ); Tue, 2 Jun 2015 02:03:59 -0400 Received: from /spool/local by e33.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 2 Jun 2015 00:03:58 -0600 In-Reply-To: <20150602053956.GD10443@linux> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Viresh Kumar 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 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