From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC v3] cpufreq: Make sure frequency transitions are serialized Date: Wed, 19 Mar 2014 20:18:54 +0530 Message-ID: <5329AE56.8080204@linux.vnet.ibm.com> References: <2efc621827cbd96a05a3d34075154974b4816ecd.1394782795.git.viresh.kumar@linaro.org> <532840FD.308@linux.vnet.ibm.com> <53296870.5010505@linux.vnet.ibm.com> <53298A7D.3080400@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: cpufreq-owner@vger.kernel.org To: Viresh Kumar Cc: "Rafael J. Wysocki" , Lists linaro-kernel , "cpufreq@vger.kernel.org" , "linux-pm@vger.kernel.org" , Linux Kernel Mailing List , Amit Daniel List-Id: linux-pm@vger.kernel.org On 03/19/2014 07:05 PM, Viresh Kumar wrote: > On 19 March 2014 17:45, Srivatsa S. Bhat > wrote: >> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h >> + bool transition_ongoing; /* Tracks transition status */ >> + struct mutex transition_lock; >> + wait_queue_head_t transition_wait; > > Similar to what I have done in my last version, why do you need > transition_ongoing and transition_wait? Simply work with > transition_lock? i.e. Acquire it for the complete transition sequence. > We *can't* acquire it for the complete transition sequence in case of drivers that do asynchronous notification, because PRECHANGE is done in one thread and POSTCHANGE is done in a totally different thread! You can't acquire a lock in one task and release it in a different task. That would be a fundamental violation of locking. That's why I introduced the wait queue to help us create a "flow" which encompasses 2 different, but co-ordinating tasks. You simply can't do that elegantly by using plain locks alone. Regards, Srivatsa S. Bhat