On 30 January 2015 at 07:51, ethan zhao ethan.zhao@oracle.com wrote:
My reasoning of why your observation doesn't fit here:
Copying from your earlier mail..
Thread A: Workqueue: kacpi_notify
acpi_processor_notify() acpi_processor_ppc_has_changed() cpufreq_update_policy() cpufreq_cpu_get() kobject_get()
This tries to increment the count and the warning you have mentioned happen because:
WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
i.e. even after incrementing the count, it is < 2. Which I believe will be
- Which means that we have tried to do kobject_get() on a kobject
for which kobject_put() is already done.
Thread B: xenbus_thread()
xenbus_thread() msg->u.watch.handle->callback() handle_vcpu_hotplug_event() vcpu_hotplug() cpu_down() __cpu_notify(CPU_DOWN_PREPARE..) cpufreq_cpu_callback() __cpufreq_remove_dev_prepare() update_policy_cpu() kobject_move()
Okay, where is the race or kobject_put() here ? We are just moving the kobject and it has nothing to do with the refcount of kobject.
Why do you see its a race ?
I mean the policy->cpu has been changed, that CPU is about to be down, Thread A continue to get and update the policy for it blindly, that is what I Say 'race', not the refcount itself.
First of all, the WARN you had in your patch doesn't have anything to do with the so-called race you just define. Its because of the reason I defined earlier.
Second, what if policy->cpu is getting updated? A policy manages a group of CPUs, not a single cpu. And there still are other CPUs online for that policy and so kobject_get() for that policy->kobj is perfectly valid.