mac80211: add pre-RCU-sync sta removal driver operation

Currently, mac80211 allows drivers to keep RCU-protected station
references that are cleared when the station is removed from the
driver and consequently needs to synchronize twice, once before
removing the station from the driver (so it can guarantee that
the station is no longer used in TX towards the driver) and once
after the station is removed from the driver.

Add a new pre-RCU-synchronisation station removal operation to
the API to allow drivers to clear/invalidate their RCU-protected
station pointers before the RCU synchronisation.

This will allow removing the second synchronisation by changing
the driver API so that the driver may no longer assume a valid
RCU-protected pointer after sta_remove/sta_state returns.

The alternative to this would be to synchronize_rcu() in all the
drivers that currently rely on this behaviour (only iwlmvm) but
that would defeat the purpose.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

3 files changed, 33 insertions, 19 deletions

diff --git a/net/mac80211/driver-ops.h b/net/mac80211/driver-ops.h
index f98059a45aa..ef8b385eff0 100644
--- a/net/mac80211/driver-ops.h
+++ b/net/mac80211/driver-ops.h
@@ -534,6 +534,22 @@ static inline void drv_sta_remove_debugfs(struct ieee80211_local *local,
 }
 #endif
 
+static inline void drv_sta_pre_rcu_remove(struct ieee80211_local *local,
+					  struct ieee80211_sub_if_data *sdata,
+					  struct sta_info *sta)
+{
+	might_sleep();
+
+	sdata = get_bss_sdata(sdata);
+	check_sdata_in_driver(sdata);
+
+	trace_drv_sta_pre_rcu_remove(local, sdata, &sta->sta);
+	if (local->ops->sta_pre_rcu_remove)
+		local->ops->sta_pre_rcu_remove(&local->hw, &sdata->vif,
+					       &sta->sta);
+	trace_drv_return_void(local);
+}
+
 static inline __must_check
 int drv_sta_state(struct ieee80211_local *local,
 		  struct ieee80211_sub_if_data *sdata,
diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index 8ae37f60d60..1e147742ecc 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -873,6 +873,8 @@ int __must_check __sta_info_destroy(struct sta_info *sta)
 
 	list_del_rcu(&sta->list);
 
+	drv_sta_pre_rcu_remove(local, sta->sdata, sta);
+
 	/* this always calls synchronize_net() */
 	ieee80211_free_sta_keys(local, sta);
 
diff --git a/net/mac80211/trace.h b/net/mac80211/trace.h
index 854bc594312..3a669d7ec7a 100644
--- a/net/mac80211/trace.h
+++ b/net/mac80211/trace.h
@@ -766,7 +766,7 @@ TRACE_EVENT(drv_sta_rc_update,
 	)
 );
 
-TRACE_EVENT(drv_sta_add,
+DECLARE_EVENT_CLASS(sta_event,
 	TP_PROTO(struct ieee80211_local *local,
 		 struct ieee80211_sub_if_data *sdata,
 		 struct ieee80211_sta *sta),
@@ -791,29 +791,25 @@ TRACE_EVENT(drv_sta_add,
 	)
 );
 
-TRACE_EVENT(drv_sta_remove,
+DEFINE_EVENT(sta_event, drv_sta_add,
 	TP_PROTO(struct ieee80211_local *local,
 		 struct ieee80211_sub_if_data *sdata,
 		 struct ieee80211_sta *sta),
+	TP_ARGS(local, sdata, sta)
+);
 
-	TP_ARGS(local, sdata, sta),
-
-	TP_STRUCT__entry(
-		LOCAL_ENTRY
-		VIF_ENTRY
-		STA_ENTRY
-	),
-
-	TP_fast_assign(
-		LOCAL_ASSIGN;
-		VIF_ASSIGN;
-		STA_ASSIGN;
-	),
+DEFINE_EVENT(sta_event, drv_sta_remove,
+	TP_PROTO(struct ieee80211_local *local,
+		 struct ieee80211_sub_if_data *sdata,
+		 struct ieee80211_sta *sta),
+	TP_ARGS(local, sdata, sta)
+);
 
-	TP_printk(
-		LOCAL_PR_FMT  VIF_PR_FMT  STA_PR_FMT,
-		LOCAL_PR_ARG, VIF_PR_ARG, STA_PR_ARG
-	)
+DEFINE_EVENT(sta_event, drv_sta_pre_rcu_remove,
+	TP_PROTO(struct ieee80211_local *local,
+		 struct ieee80211_sub_if_data *sdata,
+		 struct ieee80211_sta *sta),
+	TP_ARGS(local, sdata, sta)
 );
 
 TRACE_EVENT(drv_conf_tx,