This adds new pernet_operations::async flag to indicate operations,
which ->init(), ->exit() and ->exit_batch() methods are allowed
to be executed in parallel with the methods of any other pernet_operations.

When there are only asynchronous pernet_operations in the system,
net_mutex won't be taken for a net construction and destruction.

Also, remove BUG_ON(mutex_is_locked()) from net_assign_generic()
without replacing with the equivalent net_sem check, as there is
one more lockdep assert below.

v3: Add comment near net_mutex.

Suggested-by: Eric W. Biederman <ebied...@xmission.com>
Signed-off-by: Kirill Tkhai <ktk...@virtuozzo.com>
Acked-by: Andrei Vagin <ava...@virtuozzo.com>
---
 include/net/net_namespace.h |    6 ++++++
 net/core/net_namespace.c    |   30 ++++++++++++++++++++----------
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index f306b2aa15a4..9158ec1ad06f 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -313,6 +313,12 @@ struct pernet_operations {
        void (*exit_batch)(struct list_head *net_exit_list);
        unsigned int *id;
        size_t size;
+       /*
+        * Indicates above methods are allowed to be executed in parallel
+        * with methods of any other pernet_operations, i.e. they are not
+        * need synchronization via net_mutex.
+        */
+       bool async;
 };
 
 /*
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index f8453c438798..2a01ff32d9c7 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -29,6 +29,7 @@
 
 static LIST_HEAD(pernet_list);
 static struct list_head *first_device = &pernet_list;
+/* Used only if there are !async pernet_operations registered */
 DEFINE_MUTEX(net_mutex);
 
 LIST_HEAD(net_namespace_list);
@@ -41,8 +42,9 @@ struct net init_net = {
 EXPORT_SYMBOL(init_net);
 
 static bool init_net_initialized;
+static unsigned nr_sync_pernet_ops;
 /*
- * net_sem: protects: pernet_list, net_generic_ids,
+ * net_sem: protects: pernet_list, net_generic_ids, nr_sync_pernet_ops,
  * init_net_initialized and first_device pointer.
  */
 DECLARE_RWSEM(net_sem);
@@ -70,11 +72,10 @@ static int net_assign_generic(struct net *net, unsigned int 
id, void *data)
 {
        struct net_generic *ng, *old_ng;
 
-       BUG_ON(!mutex_is_locked(&net_mutex));
        BUG_ON(id < MIN_PERNET_OPS_ID);
 
        old_ng = rcu_dereference_protected(net->gen,
-                                          lockdep_is_held(&net_mutex));
+                                          lockdep_is_held(&net_sem));
        if (old_ng->s.len > id) {
                old_ng->ptr[id] = data;
                return 0;
@@ -426,11 +427,14 @@ struct net *copy_net_ns(unsigned long flags,
        rv = down_read_killable(&net_sem);
        if (rv < 0)
                goto put_userns;
-       rv = mutex_lock_killable(&net_mutex);
-       if (rv < 0)
-               goto up_read;
+       if (nr_sync_pernet_ops) {
+               rv = mutex_lock_killable(&net_mutex);
+               if (rv < 0)
+                       goto up_read;
+       }
        rv = setup_net(net, user_ns);
-       mutex_unlock(&net_mutex);
+       if (nr_sync_pernet_ops)
+               mutex_unlock(&net_mutex);
 up_read:
        up_read(&net_sem);
        if (rv < 0) {
@@ -487,7 +491,8 @@ static void cleanup_net(struct work_struct *work)
        spin_unlock_irq(&cleanup_list_lock);
 
        down_read(&net_sem);
-       mutex_lock(&net_mutex);
+       if (nr_sync_pernet_ops)
+               mutex_lock(&net_mutex);
 
        /* Don't let anyone else find us. */
        rtnl_lock();
@@ -522,7 +527,8 @@ static void cleanup_net(struct work_struct *work)
        list_for_each_entry_reverse(ops, &pernet_list, list)
                ops_exit_list(ops, &net_exit_list);
 
-       mutex_unlock(&net_mutex);
+       if (nr_sync_pernet_ops)
+               mutex_unlock(&net_mutex);
 
        /* Free the net generic variables */
        list_for_each_entry_reverse(ops, &pernet_list, list)
@@ -994,6 +1000,9 @@ static int register_pernet_operations(struct list_head 
*list,
                rcu_barrier();
                if (ops->id)
                        ida_remove(&net_generic_ids, *ops->id);
+       } else if (!ops->async) {
+               pr_info_once("Pernet operations %ps are sync.\n", ops);
+               nr_sync_pernet_ops++;
        }
 
        return error;
@@ -1001,7 +1010,8 @@ static int register_pernet_operations(struct list_head 
*list,
 
 static void unregister_pernet_operations(struct pernet_operations *ops)
 {
-       
+       if (!ops->async)
+               BUG_ON(nr_sync_pernet_ops-- == 0);
        __unregister_pernet_operations(ops);
        rcu_barrier();
        if (ops->id)

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