From: Venkat Chimata <venkata@shasta.cloud>
OpenWiFi's ucentral agent periodically (typically 120 seconds)
issues nl80211 call to get associated client list from the
WLAN driver. Somehow this operation was causing tx/rx delays
sometimes and the video calls on connected clients are experiencing
jitter. The associated client list was protected by
a mutex lock. I saw that ieee80211_check_fast_xmit_all uses
rcu_read_lock and rcu_read_unlock to iterate through sta_list.
I took it as a refernce and changed the lock to rcu_read lock
from mutex.
Also saw this this comment just above sta_mutex declaration.
/* Station data */
/*
* The mutex only protects the list, hash table and
* counter, reads are done with RCU.
*/
struct mutex sta_mtx;
Hence tried changing mutex_lock(/unlock) in ieee80211_dump_station
to rcu_read_lock(/unlock) and it resolved the jitter issue in the
video calls.
Tests:
We had this issue show up consistently and the patch fixed the issue.
We spent a good part of 2 weeks following up on this and with this
fix, the video calls are smooth.
Also tested if this could cause any crashes with below mentioned
process.
1. Connect 3 clients
2. A script running dumping clients in an infinite loop
3. Continuously disconnect / connect one client to see if
there is any crash. No crash was observed.
---
v2:
* Resolve warning
net/mac80211/cfg.c:858:33: warning:
unused variable 'local' [-Wunused-variable]
---
Signed-off-by: Venkat Chimata <venkata@shasta.cloud>
---
net/mac80211/cfg.c | 7 ++-----
net/mac80211/sta_info.c | 3 +--
2 files changed, 3 insertions(+), 7 deletions(-)
diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
index 6a8350d..a005364 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -790,20 +790,17 @@ static int ieee80211_dump_station(struct wiphy *wiphy, struct net_device *dev,
int idx, u8 *mac, struct station_info *sinfo)
{
struct ieee80211_sub_if_data *sdata = IEEE80211_DEV_TO_SUB_IF(dev);
- struct ieee80211_local *local = sdata->local;
struct sta_info *sta;
int ret = -ENOENT;
- mutex_lock(&local->sta_mtx);
-
+ rcu_read_lock();
sta = sta_info_get_by_idx(sdata, idx);
if (sta) {
ret = 0;
memcpy(mac, sta->sta.addr, ETH_ALEN);
sta_set_sinfo(sta, sinfo, true);
}
-
- mutex_unlock(&local->sta_mtx);
+ rcu_read_unlock();
return ret;
}
diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index 550a610..af6fa75 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -231,8 +231,7 @@ struct sta_info *sta_info_get_by_idx(struct ieee80211_sub_if_data *sdata,
struct sta_info *sta;
int i = 0;
- list_for_each_entry_rcu(sta, &local->sta_list, list,
- lockdep_is_held(&local->sta_mtx)) {
+ list_for_each_entry_rcu(sta, &local->sta_list, list) {
if (sdata != sta->sdata)
continue;
if (i < idx) {
--
2.34.1
