Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752812AbaD2Ezq (ORCPT ); Tue, 29 Apr 2014 00:55:46 -0400 Received: from mail-ob0-f177.google.com ([209.85.214.177]:39865 "EHLO mail-ob0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751450AbaD2Ezo (ORCPT ); Tue, 29 Apr 2014 00:55:44 -0400 MIME-Version: 1.0 In-Reply-To: References: <20140428185331.28755.899.stgit@srivatsabhat.in.ibm.com> <20140428185507.28755.6483.stgit@srivatsabhat.in.ibm.com> Date: Tue, 29 Apr 2014 10:25:43 +0530 Message-ID: Subject: Re: [PATCH v2 5/5] cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end From: Viresh Kumar To: "Srivatsa S. Bhat" Cc: "Rafael J. Wysocki" , Meelis Roos , "cpufreq@vger.kernel.org" , "linux-pm@vger.kernel.org" , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29 April 2014 10:21, Viresh Kumar wrote: > Nice effort. > > On 29 April 2014 00:25, Srivatsa S. Bhat > wrote: >> Now all such drivers have been fixed, but debugging this issue was not >> very straight-forward (even lockdep didn't catch this). So let us add a >> debug infrastructure to the cpufreq core to catch such issues more easily >> in the future. > > BUT, I am not sure if we really need it :( > > I think we just got into the 'barrier' stuff as we had some doubts about it > earlier and were quite sure that nothing else could go wrong. Otherwise > the only problem could have been present was the second queuing > from the same thread. And we will surely get stuck if that happens and > we can't just miss that error.. > >> Scenario 1: (Deadlock-free) >> ---------- >> >> Task A Task B >> >> /* 1st freq transition */ >> Invoke _begin() { >> ... >> ... >> } >> >> Change the frequency >> >> Got interrupt for successful >> change of frequency. >> >> /* 1st freq transition */ >> Invoke _end() { >> ... >> ... >> /* 2nd freq transition */ ... >> Invoke _begin() { ... >> ... //waiting for B ... >> ... //to finish _end() } >> ... >> ... >> } >> >> >> This scenario is actually deadlock-free because Task A can wait inside the >> second call to _begin() without self-deadlocking, because it is the >> responsibility of Task B to finish the first sequence by invoking the >> corresponding _end(). WTF, I was writing my mail and it just got send due to some stupid combination of keys :( .. Sorry. Also, this might not work as expected. Consider this scenario: /* 1st freq transition */ Invoke _begin() { ... ... } Start Change of frequency and return back as there is no end from same thread. /* 2nd freq transition as there is nobody stopping us */ Invoke _begin() { ... //waiting for B ... //to finish _end() ... ... } Got interrupt for successful change of frequency. /* 1st freq transition */ Invoke _end() { ... ... } And your patch will probably break this ? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/