Re: [Patch] cpufreq: fix a deadlock during shutting down

[Xiaotian Feng <xtfeng@gmail.com>]

On Tue, Feb 9, 2010 at 5:10 PM, Amerigo Wang <amwang@redhat.com> wrote:
>
> Benjamin reported that, the machine deadlocks right after printing the
> following when doing a shutdown:
>
> halt/4071 is trying to acquire lock:
>  (s_active){++++.+}, at: [<c0000000001ef868>] .sysfs_addrm_finish+0x58/0xc0
>
> but task is already holding lock:
>  (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at: [<c0000000004cd6ac>] .lock_policy_rwsem_write+0x84/0xf4
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> <nothing else ... machine deadlocked here>
>
>
> This is because we are trying to kobject_put() a kobject while
> we are holding cpu policy rwsem. So just move kobject_put()
> down after releasing the rwsem.
>
> Totally untested.
>
> Reported-by: Xiaotian Feng <xtfeng@gmail.com>
> Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Signed-off-by: WANG Cong <amwang@redhat.com>
> Cc: Dave Jones <davej@redhat.com>
> Cc: Thomas Renninger <trenn@suse.de>
> Cc: Prarit Bhargava <prarit@redhat.com>
> Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
>
> ---
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 67bc2ec..222b35f 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1113,6 +1113,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
>        unsigned int cpu = sys_dev->id;
>        unsigned long flags;
>        struct cpufreq_policy *data;
> +       struct kobject *kobj;
>  #ifdef CONFIG_SMP
>        struct sys_device *cpu_sys_dev;
>        unsigned int j;
> @@ -1192,7 +1193,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
>        if (cpufreq_driver->target)
>                __cpufreq_governor(data, CPUFREQ_GOV_STOP);
>
> -       kobject_put(&data->kobj);
> +       kobj = &data->kobj;
>
>        /* we need to make sure that the underlying kobj is actually
>         * not referenced anymore by anybody before we proceed with

Then kernel will wait_for_completion(&data->kobj_unregister); forever.....

> @@ -1207,6 +1208,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
>
>        unlock_policy_rwsem_write(cpu);
>
> +       kobject_put(kobj);
>        free_cpumask_var(data->related_cpus);
>        free_cpumask_var(data->cpus);
>        kfree(data);
>