cfg80211: fix circular lock dependency (1)

cfg80211: fix circular lock dependency (1)

Luis reported this lockdep complaint, that he had also
reported earlier but when trying to analyse I had been
locking at the wrong code, and never saw the problem:

(slightly abridged)
=======================================================
[ INFO: possible circular locking dependency detected ]
2.6.31-rc4-wl #6
-------------------------------------------------------
wpa_supplicant/3799 is trying to acquire lock:
 (cfg80211_mutex){+.+.+.}, at: [<ffffffffa009246a>] cfg80211_get_dev_from_ifindex+0x1a/0x90 [cfg80211]

but task is already holding lock:
 (rtnl_mutex){+.+.+.}, at: [<ffffffff81400ff2>] rtnl_lock+0x12/0x20

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (rtnl_mutex){+.+.+.}:
       [<ffffffff810857b6>] __lock_acquire+0xd76/0x12b0
       [<ffffffff81085dd3>] lock_acquire+0xe3/0x120
       [<ffffffff814ee7a4>] mutex_lock_nested+0x44/0x350
       [<ffffffff81400ff2>] rtnl_lock+0x12/0x20
       [<ffffffffa009f6a5>] nl80211_send_reg_change_event+0x1f5/0x2a0 [cfg80211]
       [<ffffffffa009529e>] set_regdom+0x28e/0x4c0 [cfg80211]

-> #0 (cfg80211_mutex){+.+.+.}:
       [<ffffffff8108587b>] __lock_acquire+0xe3b/0x12b0
       [<ffffffff81085dd3>] lock_acquire+0xe3/0x120
       [<ffffffff814ee7a4>] mutex_lock_nested+0x44/0x350
       [<ffffffffa009246a>] cfg80211_get_dev_from_ifindex+0x1a/0x90 [cfg80211]
       [<ffffffffa009813f>] get_rdev_dev_by_info_ifindex+0x6f/0xa0 [cfg80211]
       [<ffffffffa009b12b>] nl80211_set_interface+0x3b/0x260 [cfg80211]

When looking at the correct code, the problem is quite
obvious. I'm not entirely sure which code paths lead
here, so until I can analyse it better let's just use
RCU to avoid the problem.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>