genetlink: release cb_lock before requesting additional module

Requesting external module with cb_lock taken can result in
the deadlock like showed below:

[ 2458.111347] Showing all locks held in the system:
[ 2458.111347] 1 lock held by NetworkManager/582:
[ 2458.111347]  #0:  (cb_lock){++++++}, at: [<ffffffff8162bc79>] genl_rcv+0x19/0x40
[ 2458.111347] 1 lock held by modprobe/603:
[ 2458.111347]  #0:  (cb_lock){++++++}, at: [<ffffffff8162baa5>] genl_lock_all+0x15/0x30

[ 2461.579457] SysRq : Show Blocked State
[ 2461.580103]   task                        PC stack   pid father
[ 2461.580103] NetworkManager  D ffff880034b84500  4040   582      1 0x00000080
[ 2461.580103]  ffff8800197ff720 0000000000000046 00000000001d5340 ffff8800197fffd8
[ 2461.580103]  ffff8800197fffd8 00000000001d5340 ffff880019631700 7fffffffffffffff
[ 2461.580103]  ffff8800197ff880 ffff8800197ff878 ffff880019631700 ffff880019631700
[ 2461.580103] Call Trace:
[ 2461.580103]  [<ffffffff817355f9>] schedule+0x29/0x70
[ 2461.580103]  [<ffffffff81731ad1>] schedule_timeout+0x1c1/0x360
[ 2461.580103]  [<ffffffff810e69eb>] ? mark_held_locks+0xbb/0x140
[ 2461.580103]  [<ffffffff817377ac>] ? _raw_spin_unlock_irq+0x2c/0x50
[ 2461.580103]  [<ffffffff810e6b6d>] ? trace_hardirqs_on_caller+0xfd/0x1c0
[ 2461.580103]  [<ffffffff81736398>] wait_for_completion_killable+0xe8/0x170
[ 2461.580103]  [<ffffffff810b7fa0>] ? wake_up_state+0x20/0x20
[ 2461.580103]  [<ffffffff81095825>] call_usermodehelper_exec+0x1a5/0x210
[ 2461.580103]  [<ffffffff817362ed>] ? wait_for_completion_killable+0x3d/0x170
[ 2461.580103]  [<ffffffff81095cc3>] __request_module+0x1b3/0x370
[ 2461.580103]  [<ffffffff810e6b6d>] ? trace_hardirqs_on_caller+0xfd/0x1c0
[ 2461.580103]  [<ffffffff8162c5c9>] ctrl_getfamily+0x159/0x190
[ 2461.580103]  [<ffffffff8162d8a4>] genl_family_rcv_msg+0x1f4/0x2e0
[ 2461.580103]  [<ffffffff8162d990>] ? genl_family_rcv_msg+0x2e0/0x2e0
[ 2461.580103]  [<ffffffff8162da1e>] genl_rcv_msg+0x8e/0xd0
[ 2461.580103]  [<ffffffff8162b729>] netlink_rcv_skb+0xa9/0xc0
[ 2461.580103]  [<ffffffff8162bc88>] genl_rcv+0x28/0x40
[ 2461.580103]  [<ffffffff8162ad6d>] netlink_unicast+0xdd/0x190
[ 2461.580103]  [<ffffffff8162b149>] netlink_sendmsg+0x329/0x750
[ 2461.580103]  [<ffffffff815db849>] sock_sendmsg+0x99/0xd0
[ 2461.580103]  [<ffffffff810bb58f>] ? local_clock+0x5f/0x70
[ 2461.580103]  [<ffffffff810e96e8>] ? lock_release_non_nested+0x308/0x350
[ 2461.580103]  [<ffffffff815dbc6e>] ___sys_sendmsg+0x39e/0x3b0
[ 2461.580103]  [<ffffffff810565af>] ? kvm_clock_read+0x2f/0x50
[ 2461.580103]  [<ffffffff810218b9>] ? sched_clock+0x9/0x10
[ 2461.580103]  [<ffffffff810bb2bd>] ? sched_clock_local+0x1d/0x80
[ 2461.580103]  [<ffffffff810bb448>] ? sched_clock_cpu+0xa8/0x100
[ 2461.580103]  [<ffffffff810e33ad>] ? trace_hardirqs_off+0xd/0x10
[ 2461.580103]  [<ffffffff810bb58f>] ? local_clock+0x5f/0x70
[ 2461.580103]  [<ffffffff810e3f7f>] ? lock_release_holdtime.part.28+0xf/0x1a0
[ 2461.580103]  [<ffffffff8120fec9>] ? fget_light+0xf9/0x510
[ 2461.580103]  [<ffffffff8120fe0c>] ? fget_light+0x3c/0x510
[ 2461.580103]  [<ffffffff815dd1d2>] __sys_sendmsg+0x42/0x80
[ 2461.580103]  [<ffffffff815dd222>] SyS_sendmsg+0x12/0x20
[ 2461.580103]  [<ffffffff81741ad9>] system_call_fastpath+0x16/0x1b
[ 2461.580103] modprobe        D ffff88000f2c8000  4632   603    602 0x00000080
[ 2461.580103]  ffff88000f04fba8 0000000000000046 00000000001d5340 ffff88000f04ffd8
[ 2461.580103]  ffff88000f04ffd8 00000000001d5340 ffff8800377d4500 ffff8800377d4500
[ 2461.580103]  ffffffff81d0b260 ffffffff81d0b268 ffffffff00000000 ffffffff81d0b2b0
[ 2461.580103] Call Trace:
[ 2461.580103]  [<ffffffff817355f9>] schedule+0x29/0x70
[ 2461.580103]  [<ffffffff81736d4d>] rwsem_down_write_failed+0xed/0x1a0
[ 2461.580103]  [<ffffffff810bb200>] ? update_cpu_load_active+0x10/0xb0
[ 2461.580103]  [<ffffffff8137b473>] call_rwsem_down_write_failed+0x13/0x20
[ 2461.580103]  [<ffffffff8173492d>] ? down_write+0x9d/0xb2
[ 2461.580103]  [<ffffffff8162baa5>] ? genl_lock_all+0x15/0x30
[ 2461.580103]  [<ffffffff8162baa5>] genl_lock_all+0x15/0x30
[ 2461.580103]  [<ffffffff8162cbb3>] genl_register_family+0x53/0x1f0
[ 2461.580103]  [<ffffffffa01dc000>] ? 0xffffffffa01dbfff
[ 2461.580103]  [<ffffffff8162d650>] genl_register_family_with_ops+0x20/0x80
[ 2461.580103]  [<ffffffffa01dc000>] ? 0xffffffffa01dbfff
[ 2461.580103]  [<ffffffffa017fe84>] nl80211_init+0x24/0xf0 [cfg80211]
[ 2461.580103]  [<ffffffffa01dc000>] ? 0xffffffffa01dbfff
[ 2461.580103]  [<ffffffffa01dc043>] cfg80211_init+0x43/0xdb [cfg80211]
[ 2461.580103]  [<ffffffff810020fa>] do_one_initcall+0xfa/0x1b0
[ 2461.580103]  [<ffffffff8105cb93>] ? set_memory_nx+0x43/0x50
[ 2461.580103]  [<ffffffff810f75af>] load_module+0x1c6f/0x27f0
[ 2461.580103]  [<ffffffff810f2c90>] ? store_uevent+0x40/0x40
[ 2461.580103]  [<ffffffff810f82c6>] SyS_finit_module+0x86/0xb0
[ 2461.580103]  [<ffffffff81741ad9>] system_call_fastpath+0x16/0x1b
[ 2461.580103] Sched Debug Version: v0.10, 3.11.0-0.rc1.git4.1.fc20.x86_64 #1

Problem start to happen after adding net-pf-16-proto-16-family-nl80211
alias name to cfg80211 module by below commit (though that commit
itself is perfectly fine):

commit fb4e156886ce6e8309e912d8b370d192330d19d3
Author: Marcel Holtmann <marcel@holtmann.org>
Date:   Sun Apr 28 16:22:06 2013 -0700

    nl80211: Add generic netlink module alias for cfg80211/nl80211

Reported-and-tested-by: Jeff Layton <jlayton@redhat.com>
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Reviewed-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -877,8 +877,10 @@ static int ctrl_getfamily(struct sk_buff *skb, struct genl_info *info)
 #ifdef CONFIG_MODULES
 		if (res == NULL) {
 			genl_unlock();
+			up_read(&cb_lock);
 			request_module("net-pf-%d-proto-%d-family-%s",
 				       PF_NETLINK, NETLINK_GENERIC, name);
+			down_read(&cb_lock);
 			genl_lock();
 			res = genl_family_find_byname(name);
 		}