Re: [PATCH v2 03/31] net: Introduce net_sem for protection of pernet_list

[Kirill Tkhai]

On 17.01.2018 23:04, Andrei Vagin wrote:
> On Mon, Nov 20, 2017 at 09:32:34PM +0300, Kirill Tkhai wrote:
>> Curently mutex is used to protect pernet operations list. It makes
>> cleanup_net() to execute ->exit methods of the same operations set,
>> which was used on the time of ->init, even after net namespace is
>> unlinked from net_namespace_list.
>>
>> But the problem is it's need to synchronize_rcu() after net is removed
>> from net_namespace_list():
>>
>> Destroy net_ns:
>> cleanup_net()
>>   mutex_lock(&net_mutex)
>>   list_del_rcu(&net->list)
>>   synchronize_rcu()                                  <--- Sleep there for ages
>>   list_for_each_entry_reverse(ops, &pernet_list, list)
>>     ops_exit_list(ops, &net_exit_list)
>>   list_for_each_entry_reverse(ops, &pernet_list, list)
>>     ops_free_list(ops, &net_exit_list)
>>   mutex_unlock(&net_mutex)
>>
>> This primitive is not fast, especially on the systems with many processors
>> and/or when preemptible RCU is enabled in config. So, all the time, while
>> cleanup_net() is waiting for RCU grace period, creation of new net namespaces
>> is not possible, the tasks, who makes it, are sleeping on the same mutex:
>>
>> Create net_ns:
>> copy_net_ns()
>>   mutex_lock_killable(&net_mutex)                    <--- Sleep there for ages
>>
>> I observed 20-30 seconds hangs of "unshare -n" on ordinary 8-cpu laptop
>> with preemptible RCU enabled.
>>
>> The solution is to convert net_mutex to the rw_semaphore and add small locks
>> to really small number of pernet_operations, what really need them. Then,
>> pernet_operations::init/::exit methods, modifying the net-related data,
>> will require down_read() locking only, while down_write() will be used
>> for changing pernet_list.
>>
>> This gives signify performance increase, after all patch set is applied,
>> like you may see here:
>>
>> %for i in {1..10000}; do unshare -n bash -c exit; done
>>
>> *before*
>> real 1m40,377s
>> user 0m9,672s
>> sys 0m19,928s
>>
>> *after*
>> real 0m17,007s
>> user 0m5,311s
>> sys 0m11,779
>>
>> (5.8 times faster)
>>
>> This patch starts replacing net_mutex to net_sem. It adds rw_semaphore,
>> describes the variables it protects, and makes to use where appropriate.
>> net_mutex is still present, and next patches will kick it out step-by-step.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
>> ---
>>  include/linux/rtnetlink.h |    1 +
>>  net/core/net_namespace.c  |   39 ++++++++++++++++++++++++++-------------
>>  net/core/rtnetlink.c      |    4 ++--
>>  3 files changed, 29 insertions(+), 15 deletions(-)
>>
>> diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h
>> index 2032ce2eb20b..f640fc87fe1d 100644
>> --- a/include/linux/rtnetlink.h
>> +++ b/include/linux/rtnetlink.h
>> @@ -35,6 +35,7 @@ extern int rtnl_is_locked(void);
>>  
>>  extern wait_queue_head_t netdev_unregistering_wq;
>>  extern struct mutex net_mutex;
>> +extern struct rw_semaphore net_sem;
>>  
>>  #ifdef CONFIG_PROVE_LOCKING
>>  extern bool lockdep_rtnl_is_held(void);
>> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
>> index 2e512965bf42..859dce31e37e 100644
>> --- a/net/core/net_namespace.c
>> +++ b/net/core/net_namespace.c
>> @@ -41,6 +41,11 @@ struct net init_net = {
> 
>> static LIST_HEAD(pernet_list);
>> static struct list_head *first_device = &pernet_list;
>> DEFINE_MUTEX(net_mutex);
> 
> With all patches, we still have the net_mutex, I think we need to add a
> comment, which explains why we need it. Are "sync" pernet operations
> depricated after this series? Or is it ok to have them?
 
net_mutex will leave till the time all pernet_operations are converted.
But people, who don't use unconverted operations, will have performance
profit already. Comment is not a problem :)

Thanks, Kirill
 
>>  EXPORT_SYMBOL(init_net);
>>  
>>  static bool init_net_initialized;
>> +/*
>> + * net_sem: protects: pernet_list, net_generic_ids,
>> + * init_net_initialized and first_device pointer.
>> + */
>> +DECLARE_RWSEM(net_sem);
>>  
>>  #define MIN_PERNET_OPS_ID	\
>>  	((sizeof(struct net_generic) + sizeof(void *) - 1) / sizeof(void *))
>> @@ -279,7 +284,7 @@ struct net *get_net_ns_by_id(struct net *net, int id)
>>   */
>>  static __net_init int setup_net(struct net *net, struct user_namespace *user_ns)
>>  {
>> -	/* Must be called with net_mutex held */
>> +	/* Must be called with net_sem held */
>>  	const struct pernet_operations *ops, *saved_ops;
>>  	int error = 0;
>>  	LIST_HEAD(net_exit_list);
>> @@ -411,12 +416,16 @@ struct net *copy_net_ns(unsigned long flags,
>>  	net->ucounts = ucounts;
>>  	get_user_ns(user_ns);
>>  
>> -	rv = mutex_lock_killable(&net_mutex);
>> +	rv = down_read_killable(&net_sem);
>>  	if (rv < 0)
>>  		goto put_userns;
>> -
>> +	rv = mutex_lock_killable(&net_mutex);
>> +	if (rv < 0)
>> +		goto up_read;
>>  	rv = setup_net(net, user_ns);
>>  	mutex_unlock(&net_mutex);
>> +up_read:
>> +	up_read(&net_sem);
>>  	if (rv < 0) {
>>  put_userns:
>>  		put_user_ns(user_ns);
>> @@ -443,6 +452,7 @@ static void cleanup_net(struct work_struct *work)
>>  	list_replace_init(&cleanup_list, &net_kill_list);
>>  	spin_unlock_irq(&cleanup_list_lock);
>>  
>> +	down_read(&net_sem);
>>  	mutex_lock(&net_mutex);
>>  
>>  	/* Don't let anyone else find us. */
>> @@ -484,6 +494,7 @@ static void cleanup_net(struct work_struct *work)
>>  		ops_free_list(ops, &net_exit_list);
>>  
>>  	mutex_unlock(&net_mutex);
>> +	up_read(&net_sem);
>>  
>>  	/* Ensure there are no outstanding rcu callbacks using this
>>  	 * network namespace.
>> @@ -510,8 +521,10 @@ static void cleanup_net(struct work_struct *work)
>>   */
>>  void net_ns_barrier(void)
>>  {
>> +	down_write(&net_sem);
>>  	mutex_lock(&net_mutex);
>>  	mutex_unlock(&net_mutex);
>> +	up_write(&net_sem);
>>  }
>>  EXPORT_SYMBOL(net_ns_barrier);
>>  
>> @@ -838,12 +851,12 @@ static int __init net_ns_init(void)
>>  
>>  	rcu_assign_pointer(init_net.gen, ng);
>>  
>> -	mutex_lock(&net_mutex);
>> +	down_write(&net_sem);
>>  	if (setup_net(&init_net, &init_user_ns))
>>  		panic("Could not setup the initial network namespace");
>>  
>>  	init_net_initialized = true;
>> -	mutex_unlock(&net_mutex);
>> +	up_write(&net_sem);
>>  
>>  	register_pernet_subsys(&net_ns_ops);
>>  
>> @@ -983,9 +996,9 @@ static void unregister_pernet_operations(struct pernet_operations *ops)
>>  int register_pernet_subsys(struct pernet_operations *ops)
>>  {
>>  	int error;
>> -	mutex_lock(&net_mutex);
>> +	down_write(&net_sem);
>>  	error =  register_pernet_operations(first_device, ops);
>> -	mutex_unlock(&net_mutex);
>> +	up_write(&net_sem);
>>  	return error;
>>  }
>>  EXPORT_SYMBOL_GPL(register_pernet_subsys);
>> @@ -1001,9 +1014,9 @@ EXPORT_SYMBOL_GPL(register_pernet_subsys);
>>   */
>>  void unregister_pernet_subsys(struct pernet_operations *ops)
>>  {
>> -	mutex_lock(&net_mutex);
>> +	down_write(&net_sem);
>>  	unregister_pernet_operations(ops);
>> -	mutex_unlock(&net_mutex);
>> +	up_write(&net_sem);
>>  }
>>  EXPORT_SYMBOL_GPL(unregister_pernet_subsys);
>>  
>> @@ -1029,11 +1042,11 @@ EXPORT_SYMBOL_GPL(unregister_pernet_subsys);
>>  int register_pernet_device(struct pernet_operations *ops)
>>  {
>>  	int error;
>> -	mutex_lock(&net_mutex);
>> +	down_write(&net_sem);
>>  	error = register_pernet_operations(&pernet_list, ops);
>>  	if (!error && (first_device == &pernet_list))
>>  		first_device = &ops->list;
>> -	mutex_unlock(&net_mutex);
>> +	up_write(&net_sem);
>>  	return error;
>>  }
>>  EXPORT_SYMBOL_GPL(register_pernet_device);
>> @@ -1049,11 +1062,11 @@ EXPORT_SYMBOL_GPL(register_pernet_device);
>>   */
>>  void unregister_pernet_device(struct pernet_operations *ops)
>>  {
>> -	mutex_lock(&net_mutex);
>> +	down_write(&net_sem);
>>  	if (&ops->list == first_device)
>>  		first_device = first_device->next;
>>  	unregister_pernet_operations(ops);
>> -	mutex_unlock(&net_mutex);
>> +	up_write(&net_sem);
>>  }
>>  EXPORT_SYMBOL_GPL(unregister_pernet_device);
>>  
>> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
>> index dabba2a91fc8..cb06d43c4230 100644
>> --- a/net/core/rtnetlink.c
>> +++ b/net/core/rtnetlink.c
>> @@ -390,11 +390,11 @@ static void rtnl_lock_unregistering_all(void)
>>  void rtnl_link_unregister(struct rtnl_link_ops *ops)
>>  {
>>  	/* Close the race with cleanup_net() */
>> -	mutex_lock(&net_mutex);
>> +	down_write(&net_sem);
>>  	rtnl_lock_unregistering_all();
>>  	__rtnl_link_unregister(ops);
>>  	rtnl_unlock();
>> -	mutex_unlock(&net_mutex);
>> +	up_write(&net_sem);
>>  }
>>  EXPORT_SYMBOL_GPL(rtnl_link_unregister);
>>  
>>