Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Tue, 24 May 2022 13:53:31 +0200 | Subject | Re: [PATCH v3] cpufreq: fix race on cpufreq online |
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On Tue, May 24, 2022 at 1:48 PM Rafael J. Wysocki <rafael@kernel.org> wrote: > > On Tue, May 24, 2022 at 1:29 PM Viresh Kumar <viresh.kumar@linaro.org> wrote: > > > > On 24-05-22, 13:22, Rafael J. Wysocki wrote: > > > On Tue, May 24, 2022 at 1:15 PM Viresh Kumar <viresh.kumar@linaro.org> wrote: > > > > > > > > On 13-05-22, 09:57, Viresh Kumar wrote: > > > > > On 12-05-22, 12:49, Rafael J. Wysocki wrote: > > > > > > > > Moreover, I'm not sure why the locking dance in store() is necessary. > > > > > > > > > > > > > > commit fdd320da84c6 ("cpufreq: Lock CPU online/offline in cpufreq_register_driver()") > > > > > > > > > > > > I get that, but I'm wondering if locking CPU hotplug from store() is > > > > > > needed at all. I mean, if we are in store(), we are holding an active > > > > > > reference to the policy kobject, so the policy cannot go away until we > > > > > > are done anyway. Thus it should be sufficient to use the policy rwsem > > > > > > for synchronization. > > > > > > > > > > I think after the current patchset is applied and we have the inactive > > > > > policy check in store(), we won't required the dance after all. > > > > > > > > I was writing a patch for this and then thought maybe look at mails > > > > around this time, when you sent the patch, and found the reason why we > > > > need the locking dance :) > > > > > > > > https://lore.kernel.org/lkml/20150729091136.GN7557@n2100.arm.linux.org.uk/ > > > > Actually no, this is for the lock in cpufreq_driver_register(). > > > > > Well, again, if we are in store(), we are holding a reference to the > > > policy kobject, so this is not initialization time. > > > > This is the commit which made the change. > > > > commit 4f750c930822 ("cpufreq: Synchronize the cpufreq store_*() routines with CPU hotplug") > > So this was done before the entire CPU hotplug rework and it was > useful at that time. > > The current code always runs cpufreq_set_policy() under policy->rwsem > and governors are stopped under policy->rwsem, so this particular race > cannot happen AFAICS. > > Locking CPU hotplug prevents CPUs from going away while store() is > running, but in order to run store(), the caller must hold an active > reference to the policy kobject. That prevents the policy from being > freed and so policy->rwsem can be acquired. After policy->rwsem has > been acquired, policy->cpus can be checked to determine whether or not > there are any online CPUs for the given policy (there may be none), > because policy->cpus is only manipulated under policy->rwsem. > > If a CPU that belongs to the given policy is going away, > cpufreq_offline() has to remove it from policy->cpus under > policy->rwsem, so either it has to wait for store() to release > policy->rwsem, or store() will acquire policy->rwsem after it and will > find that policy->cpus is empty.
Moreover, locking CPU hotplug doesn't actually prevent cpufreq_remove_dev() from running which can happen when the cpufreq driver is unregistered, for example.
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