From mboxrd@z Thu Jan 1 00:00:00 1970 From: Prarit Bhargava Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2] Date: Tue, 05 Aug 2014 18:20:49 -0400 Message-ID: <53E158C1.6010701@redhat.com> References: <1406634362-811-1-git-send-email-prarit@redhat.com> <2066166.pXm4lKLOID@vostro.rjw.lan> <53DA8389.80804@redhat.com> <1917362.abr2Y4p7vh@vostro.rjw.lan> <53DA8A41.2030601@redhat.com> <53DAA60B.6040802@codeaurora.org> <53DAA749.5080506@redhat.com> <53DAA95B.2040505@codeaurora.org> <53DAB038.3050007@redhat.com> <53DABFA6.6090503@codeaurora.org> <53DACA26.1000908@redhat.com> <53DAE592.2030909@codeaurora.org> <53DB6B81.6050400@redhat.com> <53DBCBE8.6010809@codeaurora.org> <53DBE764.8050109@redhat.com> <53DBEC27.7050803@codeaurora.org> <53E0B657.4070007@redhat.com> <53E1556B.5070304@codeaurora.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: Received: from mx1.redhat.com ([209.132.183.28]:49521 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753322AbaHEWUz (ORCPT ); Tue, 5 Aug 2014 18:20:55 -0400 In-Reply-To: <53E1556B.5070304@codeaurora.org> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Saravana Kannan Cc: Viresh Kumar , Stephen Boyd , "Rafael J. Wysocki" , Linux Kernel Mailing List , Lenny Szubowicz , "linux-pm@vger.kernel.org" On 08/05/2014 06:06 PM, Saravana Kannan wrote: > On 08/05/2014 03:53 AM, Viresh Kumar wrote: >> On 5 August 2014 16:17, Prarit Bhargava wrote: >>> Nope, not a stupid question. After reproducing (finally!) yesterday I've been >>> wondering the same thing. >> >> Good to know that :) >> >>> I've been looking into *exactly* this. On any platform where >>> cpu_weight(affected_cpus) == 1 for a particular cpu this lockdep trace should >>> happen. >> >>> That's what I'm wondering too. I'm going to instrument the code to find out >>> this morning. I'm wondering if this comes down to a lockdep class issue >>> (perhaps lockdep puts globally defined locks like cpufreq_global_kobject in a >>> different class?). >> >> Maybe, I tried this Hack to make this somewhat similar to the other case >> on my platform with just two CPUs: >> >> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c >> index 6f02485..6b4abac 100644 >> --- a/drivers/cpufreq/cpufreq.c >> +++ b/drivers/cpufreq/cpufreq.c >> @@ -98,7 +98,7 @@ static DEFINE_MUTEX(cpufreq_governor_mutex); >> >> bool have_governor_per_policy(void) >> { >> - return !!(cpufreq_driver->flags & CPUFREQ_HAVE_GOVERNOR_PER_POLICY); >> + return !(cpufreq_driver->flags & CPUFREQ_HAVE_GOVERNOR_PER_POLICY); >> } >> EXPORT_SYMBOL_GPL(have_governor_per_policy); >> >> >> This should result in something similar to setting that per-policy-governor >> flag (Actually I could have done that too :)), and I couldn't see that crash :( >> >> That needs more investigation now, probably we can get some champ of >> sysfs stuff like Tejun/Greg into discussion now.. > > Stephen and I looked into this. This is not a sysfs framework difference. The > reason we don't have this issue when we use global tunables is because we add > the attribute group to the cpufreq_global_kobject and that kobject doesn't have > a kobj_type ops similar to the per policy kobject. So, read/write to those > attributes do NOT go through the generic show/store ops that wrap every other > cpufreq framework attribute read/writes. > > So, none of those read/write do any kind of locking. They don't race with > POLICY_EXIT (because we remove the sysfs group first thing in POLICY_EXIT) but > might still race with START/STOPs (not sure, haven't looked closely yet). > > For example, writing to sampling_rate of ondemand governor might cause a race in > update_sampling_rate(). It could race and happen between a STOP and POLICY_EXIT > (triggered by hotplug, gov change, etc). > > So, this might be a completely separate bug that needs fixing when we don't use > per policy govs. Yeah, the show_one & store_one macros don't have any locking in them :/. Okay ... at least that isn't the issue. I spent 1/2 the day trying to figure out why diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index fa11a7d..6297c76 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -745,12 +745,14 @@ static struct attribute *default_attrs[] = { #define to_policy(k) container_of(k, struct cpufreq_policy, kobj) #define to_attr(a) container_of(a, struct freq_attr, attr) +/* PRARIT - in the CPUFREQ_HAVE_GOVERNOR_PER_POLICY, this is used */ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf) { struct cpufreq_policy *policy = to_policy(kobj); struct freq_attr *fattr = to_attr(attr); ssize_t ret; + printk("%s: kobject %p\n", __FUNCTION__, kobj); if (!down_read_trylock(&cpufreq_rwsem)) return -EINVAL; wasn't printing the kobject line when acpi-cpufreq didn't have the CPUFREQ_HAVE_GOVERNOR_PER_POLICY flag. And I agree ... it is a bug. P.