On Sat, Feb 22, 2020 at 01:53:25PM +0100, Johannes Berg wrote:
> On Sat, 2020-02-22 at 15:48 +0530, madhuparnabhowmik10@xxxxxxxxx wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> >
> > list_for_each_entry_rcu() has built-in RCU and lock checking.
> >
> > Pass cond argument to list_for_each_entry_rcu() to silence
> > false lockdep warning when CONFIG_PROVE_RCU_LIST is enabled
> > by default.
>
> Umm. What warning?
>
If list_for_each_entry_rcu() is called from non rcu protection
i.e without holding rcu_read_lock, but under the protection of
a different lock then we can pass that as the condition for lockdep checking
because otherwise lockdep will complain if list_for_each_entry_rcu()
is used without rcu protection. So, if we do not pass this argument
(cond) it may lead to false lockdep warnings.
> > +++ b/net/mac80211/rx.c
> > @@ -3547,7 +3547,8 @@ static void ieee80211_rx_cooked_monitor(struct ieee80211_rx_data *rx,
> > skb->pkt_type = PACKET_OTHERHOST;
> > skb->protocol = htons(ETH_P_802_2);
> >
> > - list_for_each_entry_rcu(sdata, &local->interfaces, list) {
> > + list_for_each_entry_rcu(sdata, &local->interfaces, list,
> > + lockdep_is_held(&rx->local->rx_path_lock)) {
> > if (!ieee80211_sdata_running(sdata))
> > continue;
>
> This is not related at all.
I analysed the following traces:
ieee80211_rx_handlers() -> ieee80211_rx_handlers_result() -> ieee80211_rx_cooked_monitor()
here ieee80211_rx_handlers() is holding the rx->local->rx_path_lock and
therefore I used this for the cond argument.
If this is not right, can you help me in figuring out that which other
lock is held?
and
__ieee80211_rx_handle_packet() -> ieee80211_prepare_and_rx_handle() -> ieee80211_invoke_rx_handlers() ->
ieee80211_rx_handlers_result() -> ieee80211_rx_cooked_monitor()
Here __ieee80211_rx_handle_packet() should be called under
rcu_read_lock protection.
So this trace seems okay and no need to pass any cond.
I may have missed something, please correct me in that case.
> > @@ -4114,7 +4115,8 @@ void __ieee80211_check_fast_rx_iface(struct ieee80211_sub_if_data *sdata)
> >
> > lockdep_assert_held(&local->sta_mtx);
> >
> > - list_for_each_entry_rcu(sta, &local->sta_list, list) {
> > + list_for_each_entry_rcu(sta, &local->sta_list, list,
> > + lockdep_is_held(&local->sta_mtx)) {
>
> And this isn't even a real RCU iteration, since we _must_ hold the mutex
> here.
>
Yeah exactly, dropping _rcu (use list_for_each_entry()) would be a good option in this case.
Let me know if that is alright and I will send a new patch with all the
changes required.
Thank you,
Madhuparna
> johannes
>
