[PATCH] cpufreq: Set cpufreq_cpu_data to NULL before putting kobject

Viresh Kumar
2015-01-30T01:13:12Z

cpufreq_cpu_data is protected by cpufreq_driver_lock and one of the instances
has missed this. And as a result we get this:

------------[ 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
...
Call Trace:
[<ffffffff81661b14>] dump_stack+0x46/0x58
[<ffffffff81072b61>] warn_slowpath_common+0x81/0xa0
[<ffffffff81072c7a>] warn_slowpath_null+0x1a/0x20
[<ffffffff812e16d1>] kobject_get+0x41/0x50
[<ffffffff815262a5>] cpufreq_cpu_get+0x75/0xc0
[<ffffffff81527c3e>] cpufreq_update_policy+0x2e/0x1f0
[<ffffffff810b8cb2>] ? up+0x32/0x50
[<ffffffff81381aa9>] ? acpi_ns_get_node+0xcb/0xf2
[<ffffffff81381efd>] ? acpi_evaluate_object+0x22c/0x252
[<ffffffff813824f6>] ? acpi_get_handle+0x95/0xc0
[<ffffffff81360967>] ? acpi_has_method+0x25/0x40
[<ffffffff81391e08>] acpi_processor_ppc_has_changed+0x77/0x82
[<ffffffff81089566>] ? move_linked_works+0x66/0x90
[<ffffffff8138e8ed>] acpi_processor_notify+0x58/0xe7
[<ffffffff8137410c>] acpi_ev_notify_dispatch+0x44/0x5c
[<ffffffff8135f293>] acpi_os_execute_deferred+0x15/0x22
[<ffffffff8108c910>] process_one_work+0x160/0x410
[<ffffffff8108d05b>] worker_thread+0x11b/0x520
[<ffffffff8108cf40>] ? rescuer_thread+0x380/0x380
[<ffffffff81092421>] kthread+0xe1/0x100
[<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
[<ffffffff81669ebc>] ret_from_fork+0x7c/0xb0
[<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
---[ end trace 89e66eb9795efdf7 ]---

And here is the race:

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_POST_DEAD..)
cpufreq_cpu_callback()
__cpufreq_remove_dev_finish()
cpufreq_policy_put_kobj()
kobject_put()

cpufreq_cpu_get() gets the policy from per-cpu variable cpufreq_cpu_data under
cpufreq_driver_lock, and once it gets a valid policy it expects it to not be
freed until cpufreq_cpu_put() is called.

But the race happens when another thread puts the kobject first and updates
cpufreq_cpu_data later and that too without these locks. And so the first thread
gets a valid policy structure and before it does kobject_get() on it, the second
one does kobject_put(). And so this WARN().

Fix this by setting cpufreq_cpu_data to NULL before putting the kobject and that
too under locks.

Cc: <stable@vger.kernel.org> # 3.12+
Reported-by: Ethan Zhao <ethan.zhao@oracle.com>
Reported-and-tested-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
---
@Santosh: I have changed read locks to write locks here and so you need to test
again.

drivers/cpufreq/cpufreq.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 4473eba1d6b0..e3bf702b5588 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1409,9 +1409,10 @@ static int __cpufreq_remove_dev_finish(struct device *dev,
unsigned long flags;
struct cpufreq_policy *policy;

- read_lock_irqsave(&cpufreq_driver_lock, flags);
+ write_lock_irqsave(&cpufreq_driver_lock, flags);
policy = per_cpu(cpufreq_cpu_data, cpu);
- read_unlock_irqrestore(&cpufreq_driver_lock, flags);
+ per_cpu(cpufreq_cpu_data, cpu) = NULL;
+ write_unlock_irqrestore(&cpufreq_driver_lock, flags);

if (!policy) {
pr_debug("%s: No cpu_data found\n", __func__);
@@ -1466,7 +1467,6 @@ static int __cpufreq_remove_dev_finish(struct device *dev,
}
}

- per_cpu(cpufreq_cpu_data, cpu) = NULL;
return 0;
}

--
2.3.0.rc0.44.ga94655d