Viresh,
On 2015/1/29 16:38, Viresh Kumar wrote:
Looks like you just save my time here, Santosh has also reported a
similar race in a personal mail..
As you know, Santosh is in the same cage as me.
On 29 January 2015 at 12:12, Ethan Zhao <ethan.z...@oracle.com> wrote:
There is race observed between PPC changed notification handler worker thread
and vcpu_hotplug() called within xenbus_thread() context.
It is shown as following WARNING:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 4 at include/linux/kref.h:47
kobject_get+0x41/0x50()
Modules linked in: acpi_cpufreq(+) nfsd auth_rpcgss nfs_acl
lockd grace sunrpc xfs libcrc32c sd_mod ixgbe igb mdio ahci hwmon
...
[ 14.003548] CPU: 0 PID: 4 Comm: kworker/0:0 Not tainted
...
[ 14.003553] Workqueue: kacpi_notify acpi_os_execute_deferred
[ 14.003554] 0000000000000000 000000008c76682c ffff88094c793af8
ffffffff81661b14
[ 14.003556] 0000000000000000 0000000000000000 ffff88094c793b38
ffffffff81072b61
[ 14.003558] ffff88094c793bd8 ffff8812491f8800 0000000000000292
0000000000000000
[ 14.003560] Call Trace:
[ 14.003567] [<ffffffff81661b14>] dump_stack+0x46/0x58
[ 14.003571] [<ffffffff81072b61>] warn_slowpath_common+0x81/0xa0
[ 14.003572] [<ffffffff81072c7a>] warn_slowpath_null+0x1a/0x20
[ 14.003574] [<ffffffff812e16d1>] kobject_get+0x41/0x50
[ 14.003579] [<ffffffff815262a5>] cpufreq_cpu_get+0x75/0xc0
[ 14.003581] [<ffffffff81527c3e>] cpufreq_update_policy+0x2e/0x1f0
[ 14.003586] [<ffffffff810b8cb2>] ? up+0x32/0x50
[ 14.003589] [<ffffffff81381aa9>] ? acpi_ns_get_node+0xcb/0xf2
[ 14.003591] [<ffffffff81381efd>] ? acpi_evaluate_object+0x22c/0x252
[ 14.003593] [<ffffffff813824f6>] ? acpi_get_handle+0x95/0xc0
[ 14.003596] [<ffffffff81360967>] ? acpi_has_method+0x25/0x40
[ 14.003601] [<ffffffff81391e08>] acpi_processor_ppc_has_changed+0x77/0x82
[ 14.003604] [<ffffffff81089566>] ? move_linked_works+0x66/0x90
[ 14.003606] [<ffffffff8138e8ed>] acpi_processor_notify+0x58/0xe7
[ 14.003609] [<ffffffff8137410c>] acpi_ev_notify_dispatch+0x44/0x5c
[ 14.003611] [<ffffffff8135f293>] acpi_os_execute_deferred+0x15/0x22
[ 14.003614] [<ffffffff8108c910>] process_one_work+0x160/0x410
[ 14.003616] [<ffffffff8108d05b>] worker_thread+0x11b/0x520
[ 14.003617] [<ffffffff8108cf40>] ? rescuer_thread+0x380/0x380
[ 14.003621] [<ffffffff81092421>] kthread+0xe1/0x100
[ 14.003623] [<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
[ 14.003628] [<ffffffff81669ebc>] ret_from_fork+0x7c/0xb0
[ 14.003630] [<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
[ 14.003631] ---[ end trace 89e66eb9795efdf7 ]---
Thread A: Workqueue: kacpi_notify
acpi_processor_notify()
acpi_processor_ppc_has_changed()
cpufreq_update_policy()
cpufreq_cpu_get()
kobject_get()
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()
Where is the race ? How do you say this is racy ?
You see it. the policy had been moved to another CPU, you want the Thread A
continue to get the policy and update it ?
I am not sure if the problem is with kobject_move(), to me it looked like
the problem is with cpufreq_policy_put_kobj() and we tried to do kobject_get()
after the kobject has been freed..
I don't agree to the solution you gave, but lets first make sure what the
problem is, and then take any action against it.
Please take a look at the code I composed, I thought of it whole night.
Thanks,
Ethan
Please try this patch and let us know if it fixes it for you:
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 4473eba1d6b0..5ced9cca4822 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1411,6 +1411,7 @@ static int __cpufreq_remove_dev_finish(struct device *dev,
read_lock_irqsave(&cpufreq_driver_lock, flags);
policy = per_cpu(cpufreq_cpu_data, cpu);
+ per_cpu(cpufreq_cpu_data, cpu) = NULL;
read_unlock_irqrestore(&cpufreq_driver_lock, flags);
if (!policy) {
@@ -1466,7 +1467,6 @@ static int __cpufreq_remove_dev_finish(struct device *dev,
}
}
- per_cpu(cpufreq_cpu_data, cpu) = NULL;
return 0;
}
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