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Message-ID: <5441080F.1070900@redhat.com> Date: Fri, 17 Oct 2014 08:14:07 -0400 From: Prarit Bhargava <prarit@...hat.com> To: Viresh Kumar <viresh.kumar@...aro.org> CC: Saravana Kannan <skannan@...eaurora.org>, "Rafael J. Wysocki" <rjw@...ysocki.net>, "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>, Linux Kernel <linux-kernel@...r.kernel.org>, Robert Schöne <robert.schoene@...dresden.de> Subject: Re: Locking issues with cpufreq and sysfs On 10/16/2014 07:23 AM, Viresh Kumar wrote: > On 14 October 2014 23:54, Prarit Bhargava <prarit@...hat.com> wrote: >> Here's what I think we should do. Taking a step back, the purpose of the >> cpufreq sysfs files is to allow userspace to read current cpu frequency >> settings, and to allow userspce to modify the governor and set the max & min >> ranges for cpu frequencies. This can be done per device or for all cpus >> depending on the driver. > > Okay. > >> We have to guarantee that bothing reading and writing will always work and that >> write operations will always be atomic relative to userspace. The current > > Ok. > >> implementation of cpufreq does this through the following locks: >> >> cpufreq_driver_lock: protects the cpufreq_cpu_data array and cpufreq_driver->boost >> cpufreq_governor_lock: protects the current governor > > Its just for serialization.. > >> cpufreq_governor_mutex: protects the cpufreq_governor_list >> cpufreq_rwsem: protects the driver from being unloaded >> global_kobj_lock: protects the "cpufreq" kobject >> each policy has a rwsem (policy->rwsem): protects the cpufreq_policy struct >> each policy has a transition_lock (policy->transition): synchronizes >> frequency transitions >> >> While examining this code I was wondering exactly why we allow multiple readers >> and writers in cpufreq. I could understand if we felt that this data was >> critical; but it really isn't. A short delay here isn't that big of a deal IMO >> (if someone can produce a case where a delay would cause a serious problem I'd >> like to hear it). I don't even think it is safe in most cases to allow readers >> while cpufreq values are changing; if we're changing the governor userspace >> cannot rely on the value of (for example) cpuinfo_max_freq. > > I don't know how reader writer lock will fail and a normal lock will not. > There is only benefit of rwlock, that readers can read things while > there is nobody > writing.. > >> So I'm proposing that we move to a single threaded read/write using, if > > Okay, but how will that benefit us ? It will greatly simplify the code. The locking isn't working in this code at all right now and is causing various reported panics ... you yourself are pushing a lock patch that serializes operations -- which is causing other problems during testing. > >> possible, a single policy lock for now. We might transition this back to a >> rwsem later on, however, for the first attempt at cleaning this up I think we >> should just stick with a simple lock. In doing that, IMO we remove >> >> cpufreq_rwsem: protects the driver from being unloaded >> cpufreq_governor_lock: protects the current governor >> each policy has a rwsem (policy->rwsem): protects the cpufreq_policy struct >> >> and potentially >> >> cpufreq_driver_lock: protects the cpufreq_cpu_data array and cpufreq_driver->boost > > Not really sure, but yeah we might be able to club few of them.. > >> After looking at the way the code would be structured, I'm wondering if >> >> cpufreq_governor_mutex: protects the cpufreq_governor_list >> >> is overkill. The loading of a module should be atomic relative to the cpufreq >> code, so this lock may not be required. (Admittedly I haven't tested that...) >> >> That would leave: >> >> global_kobj_lock: protects the "cpufreq" kobject >> each policy has a transition_lock (policy->transition): synchronizes >> frequency transitions >> >> and a new lock, perhaps called policy->lock, to serialize all events. >> >> Pros: We clean all this up to a simpler single threaded model. Bugs and races >> here would be much easier to handle. We're currently putting band-aid on >> band-aids in this code ATM and it looks like we're seeing old races expanded >> or new races exposed. >> >> Cons: We lose the ability to do simultaneous reads and writes ... although >> I remain unconvinced that this would ever be safe to do. ie) If I change the >> governor while at the same time reading, for example, the current cpu >> frequency I cannot rely on that value to be valid. >> >> After that we can add some reference counting to the sysfs file accesses >> so that we can block after the sysfs removal when we change cpufreq >> governors. I think that would be trivial and that it would resolve any races >> when adding and removing governor's sysfs files. > > Not really sure, but if you solve few things with getting these bugs resolved > then we might apply your patches without any issues. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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