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>

1 files changed, 15 insertions, 19 deletions

diff --git a/net/mac80211/trace.h b/net/mac80211/trace.h
index 854bc5943127..3a669d7ec7ad 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,