nl80211_pre_doit acquires rtnl_lock and then wiphy_lock, releasing
rtnl while keeping wiphy_lock held until post_doit. With the
introduction of rx_mode_wq and its flush in netdev_run_todo, calling
rtnl_unlock here creates a circular lock dependency:

  Chain exists of:
    (wq_completion)rx_mode_wq --> rtnl_mutex --> &rdev->wiphy.mtx

   Possible unsafe locking scenario:

         CPU0                    CPU1
         ----                    ----
    lock(&rdev->wiphy.mtx);
                                 lock(rtnl_mutex);
                                 lock(&rdev->wiphy.mtx);
    lock((wq_completion)rx_mode_wq);

Switch to __rtnl_unlock to skip netdev_run_todo in nl80211_pre_doit.
This seems safe because we run before the op.

Link: http://lore.kernel.org/netdev/69b5ad67.a00a0220.3b25d1.001a.GAE@google.com (local)
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
---
 net/core/rtnetlink.c   | 1 +
 net/wireless/core.c    | 1 +
 net/wireless/nl80211.c | 2 +-
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index dad4b1054955..b1bfb4a4aedd 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c

@@ -150,6 +150,7 @@ void __rtnl_unlock(void)
 		head = next;
 	}
 }
+EXPORT_SYMBOL_NS_GPL(__rtnl_unlock, "NETDEV_INTERNAL");
 
 void rtnl_unlock(void)
 {

diff --git a/net/wireless/core.c b/net/wireless/core.c
index 23afc250bc10..a78a9b613c94 100644
--- a/net/wireless/core.c
+++ b/net/wireless/core.c

@@ -41,6 +41,7 @@ MODULE_AUTHOR("Johannes Berg");
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("wireless configuration support");
 MODULE_ALIAS_GENL_FAMILY(NL80211_GENL_NAME);
+MODULE_IMPORT_NS("NETDEV_INTERNAL");
 
 /* RCU-protected (and RTNL for writers) */
 LIST_HEAD(cfg80211_rdev_list);

diff --git a/net/wireless/nl80211.c b/net/wireless/nl80211.c
index e15cd26f3a79..f8ed8730cf1c 100644
--- a/net/wireless/nl80211.c
+++ b/net/wireless/nl80211.c

@@ -18203,7 +18203,7 @@ static int nl80211_pre_doit(const struct genl_split_ops *ops,
 		__release(&rdev->wiphy.mtx);
 	}
 	if (!(internal_flags & NL80211_FLAG_NEED_RTNL))
-		rtnl_unlock();
+		__rtnl_unlock();
 
 	return 0;
 out_unlock:

-- 
2.53.0