From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Pallipadi, Venkatesh" Subject: Re: [patch 2/3] cpufreq: Define dbs_mutex purpose and cleanup its usage Date: Thu, 25 Jun 2009 13:54:45 -0700 Message-ID: <1245963285.4534.20542.camel@localhost.localdomain> References: <20090625183354.491259000@intel.com> <20090625183601.493904000@intel.com> <20090625194648.GA24657@Krystal> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20090625194648.GA24657@Krystal> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Mathieu Desnoyers Cc: Dave Jones , "linux-kernel@vger.kernel.org" , "cpufreq@vger.kernel.org" , "kernel-testers@vger.kernel.org" , Ingo Molnar , "Rafael J. Wysocki" , Dave Young , Pekka Enberg , Thomas Renninger On Thu, 2009-06-25 at 12:46 -0700, Mathieu Desnoyers wrote: > * venkatesh.pallipadi@intel.com (venkatesh.pallipadi@intel.com) wrote: > > Commit b14893a62c73af0eca414cfed505b8c09efc613c although it was very > > much needed to cleanup ondemand timer cleanly, openup a can of worms > > related to locking dependencies in cpufreq. > > > > Patch here defines the need for dbs_mutex and cleans up its usage in > > ondemand governor. This also resolves the lockdep warnings reported here > > > > http://lkml.indiana.edu/hypermail/linux/kernel/0906.1/01925.html > > > > @@ -598,14 +593,16 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy, > > max(min_sampling_rate, > > latency * LATENCY_MULTIPLIER); > > } > > + mutex_unlock(&dbs_mutex); > > + > > dbs_timer_init(this_dbs_info); > > > > - mutex_unlock(&dbs_mutex); > > break; > > > > case CPUFREQ_GOV_STOP: > > - mutex_lock(&dbs_mutex); > > dbs_timer_exit(this_dbs_info); > > Hrm, so.. how do we protect against concurrent : > > CPUFREQ_GOV_START/CPUFREQ_GOV_STOP now ? concurrent _START _STOP across CPUs does not matter for timer_init and timer_exit. On same CPU, there cannot be two concurrent _START as upper level cpufreq will have policy_rwsem in write mode. I cannot think of a flow where _START and _STOP on same CPU is possible. However two concurrent _STOP for same CPU is still possible, as we are releasing the rwsem lock before STOP callback. "Back to drawing board" time to figure this all out.. Thanks, Venki