2020-02-22 10:23:54

by Madhuparna Bhowmik

[permalink] [raw]
Subject: [PATCH] net: mac80211: rx.c: Use built-in RCU list checking

From: Madhuparna Bhowmik <[email protected]>

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.

Signed-off-by: Madhuparna Bhowmik <[email protected]>
---
net/mac80211/rx.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index 0e05ff037672..0967bdc75938 100644
--- a/net/mac80211/rx.c
+++ 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;

@@ -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)) {
if (sdata != sta->sdata &&
(!sta->sdata->bss || sta->sdata->bss != sdata->bss))
continue;
--
2.17.1


2020-02-22 12:54:00

by Johannes Berg

[permalink] [raw]
Subject: Re: [PATCH] net: mac80211: rx.c: Use built-in RCU list checking

On Sat, 2020-02-22 at 15:48 +0530, [email protected] wrote:
> From: Madhuparna Bhowmik <[email protected]>
>
> 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?

> +++ 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.

> @@ -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.

johannes

2020-02-22 13:39:58

by Madhuparna Bhowmik

[permalink] [raw]
Subject: Re: [PATCH] net: mac80211: rx.c: Use built-in RCU list checking

On Sat, Feb 22, 2020 at 01:53:25PM +0100, Johannes Berg wrote:
> On Sat, 2020-02-22 at 15:48 +0530, [email protected] wrote:
> > From: Madhuparna Bhowmik <[email protected]>
> >
> > 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
>