Re: [PATCH] cpufreq: Fix sysfs deadlock with concurrent hotplug/frequency switch

[Rafael J. Wysocki]

On Friday, July 20, 2012, Stephen Boyd wrote:
> Running one program that continuously hotplugs and replugs a cpu
> concurrently with another program that continuously writes to the
> scaling_setspeed node eventually deadlocks with:
> 
> =============================================
> [ INFO: possible recursive locking detected ]
> 3.4.0 #37 Tainted: G        W
> ---------------------------------------------
> filemonkey/122 is trying to acquire lock:
>  (s_active#13){++++.+}, at: [<c01a3d28>] sysfs_remove_dir+0x9c/0xb4
> 
> but task is already holding lock:
>  (s_active#13){++++.+}, at: [<c01a22f0>] sysfs_write_file+0xe8/0x140
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(s_active#13);
>   lock(s_active#13);
> 
>  *** DEADLOCK ***
> 
>  May be due to missing lock nesting notation
> 
> 2 locks held by filemonkey/122:
>  #0:  (&buffer->mutex){+.+.+.}, at: [<c01a2230>] sysfs_write_file+0x28/0x140
>  #1:  (s_active#13){++++.+}, at: [<c01a22f0>] sysfs_write_file+0xe8/0x140
> 
> stack backtrace:
> [<c0014fcc>] (unwind_backtrace+0x0/0x120) from [<c00ca600>] (validate_chain+0x6f8/0x1054)
> [<c00ca600>] (validate_chain+0x6f8/0x1054) from [<c00cb778>] (__lock_acquire+0x81c/0x8d8)
> [<c00cb778>] (__lock_acquire+0x81c/0x8d8) from [<c00cb9c0>] (lock_acquire+0x18c/0x1e8)
> [<c00cb9c0>] (lock_acquire+0x18c/0x1e8) from [<c01a3ba8>] (sysfs_addrm_finish+0xd0/0x180)
> [<c01a3ba8>] (sysfs_addrm_finish+0xd0/0x180) from [<c01a3d28>] (sysfs_remove_dir+0x9c/0xb4)
> [<c01a3d28>] (sysfs_remove_dir+0x9c/0xb4) from [<c02d0e5c>] (kobject_del+0x10/0x38)
> [<c02d0e5c>] (kobject_del+0x10/0x38) from [<c02d0f74>] (kobject_release+0xf0/0x194)
> [<c02d0f74>] (kobject_release+0xf0/0x194) from [<c0565a98>] (cpufreq_cpu_put+0xc/0x24)
> [<c0565a98>] (cpufreq_cpu_put+0xc/0x24) from [<c05683f0>] (store+0x6c/0x74)
> [<c05683f0>] (store+0x6c/0x74) from [<c01a2314>] (sysfs_write_file+0x10c/0x140)
> [<c01a2314>] (sysfs_write_file+0x10c/0x140) from [<c014af44>] (vfs_write+0xb0/0x128)
> [<c014af44>] (vfs_write+0xb0/0x128) from [<c014b06c>] (sys_write+0x3c/0x68)
> [<c014b06c>] (sys_write+0x3c/0x68) from [<c000e0e0>] (ret_fast_syscall+0x0/0x3c)
> 
> This is because store() in cpufreq.c indirectly calls
> kobject_get() via cpufreq_cpu_get() and is the last one to call
> kobject_put() via cpufreq_cpu_put(). Sysfs code should not call
> kobject_get() or kobject_put() directly (see the comment around
> sysfs_schedule_callback() for more information).
> 
> Fix this deadlock by introducing two new functions:
> 
> 	struct cpufreq_policy *cpufreq_cpu_get_sysfs(unsigned int cpu)
> 	void cpufreq_cpu_put_sysfs(struct cpufreq_policy *data)
> 
> which do the same thing as cpufreq_cpu_{get,put}() but don't call
> kobject functions.
> 
> To easily trigger this deadlock you can apply a one line patch to
> the store() function in cpufreq.c

The following part of your changelog has confused Patchwork.  I guess it
will also confuse other tools, so care to describe what to do instead?

>  diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>  index a290771..62af12d 100644
>  --- a/drivers/cpufreq/cpufreq.c
>  +++ b/drivers/cpufreq/cpufreq.c
>  @@ -675,6 +675,7 @@ static ssize_t store(struct kobject *kobj
> 
>          unlock_policy_rwsem_write(policy->cpu);
>   fail:
>  +       msleep(10000);
>          cpufreq_cpu_put_sysfs(policy);
>   no_policy:
>          return ret;
> 
> and then write scaling_setspeed in one task and offline the cpu
> in another. The first task will hang and be detected by the hung
> task detector.
> 
> Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
> ---
> 
> Before you ask, I've seen the comment above cpufreq_add_dev() about
> concurrent hotplug/cpufreq.
> 
>  drivers/cpufreq/cpufreq.c | 35 +++++++++++++++++++++++++++--------
>  1 file changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 7f2f149..a290771 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -138,7 +138,7 @@ void disable_cpufreq(void)
>  static LIST_HEAD(cpufreq_governor_list);
>  static DEFINE_MUTEX(cpufreq_governor_mutex);
>  
> -struct cpufreq_policy *cpufreq_cpu_get(unsigned int cpu)
> +static struct cpufreq_policy *__cpufreq_cpu_get(unsigned int cpu, int sysfs)

I'd prefer the sysfs arg to be a bool.

>  {
>  	struct cpufreq_policy *data;
>  	unsigned long flags;
> @@ -162,7 +162,7 @@ struct cpufreq_policy *cpufreq_cpu_get(unsigned int cpu)
>  	if (!data)
>  		goto err_out_put_module;
>  
> -	if (!kobject_get(&data->kobj))
> +	if (!sysfs && !kobject_get(&data->kobj))
>  		goto err_out_put_module;
>  
>  	spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
> @@ -175,16 +175,35 @@ err_out_unlock:
>  err_out:
>  	return NULL;
>  }
> +
> +struct cpufreq_policy *cpufreq_cpu_get(unsigned int cpu)
> +{
> +	return __cpufreq_cpu_get(cpu, 0);
> +}
>  EXPORT_SYMBOL_GPL(cpufreq_cpu_get);
>  
> +static struct cpufreq_policy *cpufreq_cpu_get_sysfs(unsigned int cpu)
> +{
> +	return __cpufreq_cpu_get(cpu, 1);
> +}
>  
> -void cpufreq_cpu_put(struct cpufreq_policy *data)
> +static void __cpufreq_cpu_put(struct cpufreq_policy *data, int sysfs)
>  {
> -	kobject_put(&data->kobj);
> +	if (!sysfs)
> +		kobject_put(&data->kobj);
>  	module_put(cpufreq_driver->owner);
>  }
> +
> +void cpufreq_cpu_put(struct cpufreq_policy *data)
> +{
> +	__cpufreq_cpu_put(data, 0);
> +}
>  EXPORT_SYMBOL_GPL(cpufreq_cpu_put);
>  
> +static void cpufreq_cpu_put_sysfs(struct cpufreq_policy *data)
> +{
> +	__cpufreq_cpu_put(data, 1);
> +}
>  
>  /*********************************************************************
>   *            EXTERNALLY AFFECTING FREQUENCY CHANGES                 *
> @@ -617,7 +636,7 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
>  	struct cpufreq_policy *policy = to_policy(kobj);
>  	struct freq_attr *fattr = to_attr(attr);
>  	ssize_t ret = -EINVAL;
> -	policy = cpufreq_cpu_get(policy->cpu);
> +	policy = cpufreq_cpu_get_sysfs(policy->cpu);
>  	if (!policy)
>  		goto no_policy;
>  
> @@ -631,7 +650,7 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
>  
>  	unlock_policy_rwsem_read(policy->cpu);
>  fail:
> -	cpufreq_cpu_put(policy);
> +	cpufreq_cpu_put_sysfs(policy);
>  no_policy:
>  	return ret;
>  }
> @@ -642,7 +661,7 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>  	struct cpufreq_policy *policy = to_policy(kobj);
>  	struct freq_attr *fattr = to_attr(attr);
>  	ssize_t ret = -EINVAL;
> -	policy = cpufreq_cpu_get(policy->cpu);
> +	policy = cpufreq_cpu_get_sysfs(policy->cpu);
>  	if (!policy)
>  		goto no_policy;
>  
> @@ -656,7 +675,7 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>  
>  	unlock_policy_rwsem_write(policy->cpu);
>  fail:
> -	cpufreq_cpu_put(policy);
> +	cpufreq_cpu_put_sysfs(policy);
>  no_policy:
>  	return ret;
>  }
> 

Thanks,
Rafael
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