mac80211: mesh: fix RCU warning

ifmsh->csa is an RCU-protected pointer. The writer context
in ieee80211_mesh_finish_csa() is already mutually
exclusive with wdev->sdata.mtx, but the RCU checker did
not know this. Use rcu_dereference_protected() to avoid a
warning.

fixes the following warning:

[   12.519089] =============================
[   12.520042] WARNING: suspicious RCU usage
[   12.520652] 5.1.0-rc7-wt+ openbmc#16 Tainted: G        W
[   12.521409] -----------------------------
[   12.521972] net/mac80211/mesh.c:1223 suspicious rcu_dereference_check() usage!
[   12.522928] other info that might help us debug this:
[   12.523984] rcu_scheduler_active = 2, debug_locks = 1
[   12.524855] 5 locks held by kworker/u8:2/152:
[   12.525438]  #0: 00000000057be08c ((wq_completion)phy0){+.+.}, at: process_one_work+0x1a2/0x620
[   12.526607]  openbmc#1: 0000000059c6b07a ((work_completion)(&sdata->csa_finalize_work)){+.+.}, at: process_one_work+0x1a2/0x620
[   12.528001]  openbmc#2: 00000000f184ba7d (&wdev->mtx){+.+.}, at: ieee80211_csa_finalize_work+0x2f/0x90
[   12.529116]  openbmc#3: 00000000831a1f54 (&local->mtx){+.+.}, at: ieee80211_csa_finalize_work+0x47/0x90
[   12.530233]  openbmc#4: 00000000fd06f988 (&local->chanctx_mtx){+.+.}, at: ieee80211_csa_finalize_work+0x51/0x90

Signed-off-by: Thomas Pedersen <thomas@eero.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh.c

@@ -1220,7 +1220,8 @@ int ieee80211_mesh_finish_csa(struct ieee80211_sub_if_data *sdata)
 	ifmsh->chsw_ttl = 0;
 
 	/* Remove the CSA and MCSP elements from the beacon */
-	tmp_csa_settings = rcu_dereference(ifmsh->csa);
+	tmp_csa_settings = rcu_dereference_protected(ifmsh->csa,
+					    lockdep_is_held(&sdata->wdev.mtx));
 	RCU_INIT_POINTER(ifmsh->csa, NULL);
 	if (tmp_csa_settings)
 		kfree_rcu(tmp_csa_settings, rcu_head);
@@ -1242,6 +1243,8 @@ int ieee80211_mesh_csa_beacon(struct ieee80211_sub_if_data *sdata,
 	struct mesh_csa_settings *tmp_csa_settings;
 	int ret = 0;
 
+	lockdep_assert_held(&sdata->wdev.mtx);
+
 	tmp_csa_settings = kmalloc(sizeof(*tmp_csa_settings),
 				   GFP_ATOMIC);
 	if (!tmp_csa_settings)