net: suppress RCU lockdep false positive in sock_update_classid

> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> include/linux/cgroup.h:542 invoked rcu_dereference_check() without protection!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 1, debug_locks = 0
> 1 lock held by swapper/1:
>  #0:  (net_mutex){+.+.+.}, at: [<ffffffff813e9010>]
> register_pernet_subsys+0x1f/0x47
>
> stack backtrace:
> Pid: 1, comm: swapper Not tainted 2.6.35.4-28.fc14.x86_64 #1
> Call Trace:
>  [<ffffffff8107bd3a>] lockdep_rcu_dereference+0xaa/0xb3
>  [<ffffffff813e04b9>] sock_update_classid+0x7c/0xa2
>  [<ffffffff813e054a>] sk_alloc+0x6b/0x77
>  [<ffffffff8140b281>] __netlink_create+0x37/0xab
>  [<ffffffff813f941c>] ? rtnetlink_rcv+0x0/0x2d
>  [<ffffffff8140cee1>] netlink_kernel_create+0x74/0x19d
>  [<ffffffff8149c3ca>] ? __mutex_lock_common+0x339/0x35b
>  [<ffffffff813f7e9c>] rtnetlink_net_init+0x2e/0x48
>  [<ffffffff813e8d7a>] ops_init+0xe9/0xff
>  [<ffffffff813e8f0d>] register_pernet_operations+0xab/0x130
>  [<ffffffff813e901f>] register_pernet_subsys+0x2e/0x47
>  [<ffffffff81db7bca>] rtnetlink_init+0x53/0x102
>  [<ffffffff81db835c>] netlink_proto_init+0x126/0x143
>  [<ffffffff81db8236>] ? netlink_proto_init+0x0/0x143
>  [<ffffffff810021b8>] do_one_initcall+0x72/0x186
>  [<ffffffff81d78ebc>] kernel_init+0x23b/0x2c9
>  [<ffffffff8100aae4>] kernel_thread_helper+0x4/0x10
>  [<ffffffff8149e2d0>] ? restore_args+0x0/0x30
>  [<ffffffff81d78c81>] ? kernel_init+0x0/0x2c9
>  [<ffffffff8100aae0>] ? kernel_thread_helper+0x0/0x10

The sock_update_classid() function calls task_cls_classid(current),
but the calling task cannot go away, so there is no danger of
the associated structures disappearing.  Insert an RCU read-side
critical section to suppress the false positive.

Reported-by: Subrata Modak <subrata@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

net/core/sock.c

@@ -1078,8 +1078,11 @@ static void sk_prot_free(struct proto *prot, struct sock *sk)
 #ifdef CONFIG_CGROUPS
 void sock_update_classid(struct sock *sk)
 {
-	u32 classid = task_cls_classid(current);
+	u32 classid;
 
+	rcu_read_lock();  /* doing current task, which cannot vanish. */
+	classid = task_cls_classid(current);
+	rcu_read_unlock();
 	if (classid && classid != sk->sk_classid)
 		sk->sk_classid = classid;
 }