Re: Lockdep warning after c0feea594e058223973db94c1c32a830c9807c86

[Cong Wang <xiyou.wangcong@gmail.com>]

On Thu, Oct 13, 2022 at 10:39:08PM +0200, Jakub Sitnicki wrote:
> Hi Stan,
> 
> On Wed, Oct 12, 2022 at 02:08 PM -07, sdf@google.com wrote:
> > Hi John & Jakub,
> >
> > Upstream commit c0feea594e05 ("workqueue: don't skip lockdep work
> > dependency in cancel_work_sync()") seems to trigger the following
> > lockdep warning during test_prog's sockmap_listen:
> >
> > [  +0.003631] WARNING: possible circular locking dependency detected
> 
> [...]
> 
> > Are you ware? Any idea what's wrong?
> > Is there some stable fix I'm missing in bpf-next?
> 
> Thanks for bringing it up. I didn't know.
> 
> The mentioned commit doesn't look that fresh
> 
> commit c0feea594e058223973db94c1c32a830c9807c86
> Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date:   Fri Jul 29 13:30:23 2022 +0900
> 
>     workqueue: don't skip lockdep work dependency in cancel_work_sync()
> 
> ... but then it just landed not so long ago, which explains things:
> 
> $ git describe --contains c0feea594e058223973db94c1c32a830c9807c86 --match 'v*'
> v6.0-rc7~10^2
> 
> I've untangled the call chains leading to the potential dead-lock a
> bit. There does seem to be a window of opportunity there.
> 
> psock->work.func = sk_psock_backlog()
>   ACQUIRE psock->work_mutex
>     sk_psock_handle_skb()
>       skb_send_sock()
>         __skb_send_sock()
>           sendpage_unlocked()
>             kernel_sendpage()
>               sock->ops->sendpage = inet_sendpage()
>                 sk->sk_prot->sendpage = tcp_sendpage()
>                   ACQUIRE sk->sk_lock
>                     tcp_sendpage_locked()
>                   RELEASE sk->sk_lock
>   RELEASE psock->work_mutex
> 
> sock_map_close()
>   ACQUIRE sk->sk_lock
>   sk_psock_stop()
>     sk_psock_clear_state(psock, SK_PSOCK_TX_ENABLED)
>     cancel_work_sync()
>       __cancel_work_timer()
>         __flush_work()
>           // wait for psock->work to finish
>   RELEASE sk->sk_lock
> 
> There is no fix I know of. Need to think. Ideas welcome.
> 

Thanks for the analysis.

I wonder if we can simply move this cancel_work_sync() out of sock
lock... Something like this:

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index 48f4b645193b..70d6cb94e580 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -376,7 +376,7 @@ static inline void sk_psock_report_error(struct sk_psock *psock, int err)
 }
 
 struct sk_psock *sk_psock_init(struct sock *sk, int node);
-void sk_psock_stop(struct sk_psock *psock, bool wait);
+void sk_psock_stop(struct sk_psock *psock);
 
 #if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
 int sk_psock_init_strp(struct sock *sk, struct sk_psock *psock);
diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index ca70525621c7..ddc56660ce97 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -803,16 +803,13 @@ static void sk_psock_link_destroy(struct sk_psock *psock)
 	}
 }
 
-void sk_psock_stop(struct sk_psock *psock, bool wait)
+void sk_psock_stop(struct sk_psock *psock)
 {
 	spin_lock_bh(&psock->ingress_lock);
 	sk_psock_clear_state(psock, SK_PSOCK_TX_ENABLED);
 	sk_psock_cork_free(psock);
 	__sk_psock_zap_ingress(psock);
 	spin_unlock_bh(&psock->ingress_lock);
-
-	if (wait)
-		cancel_work_sync(&psock->work);
 }
 
 static void sk_psock_done_strp(struct sk_psock *psock);
@@ -850,7 +847,7 @@ void sk_psock_drop(struct sock *sk, struct sk_psock *psock)
 		sk_psock_stop_verdict(sk, psock);
 	write_unlock_bh(&sk->sk_callback_lock);
 
-	sk_psock_stop(psock, false);
+	sk_psock_stop(psock);
 
 	INIT_RCU_WORK(&psock->rwork, sk_psock_destroy);
 	queue_rcu_work(system_wq, &psock->rwork);
diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index a660baedd9e7..81beb16ab1eb 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -1596,7 +1596,7 @@ void sock_map_destroy(struct sock *sk)
 	saved_destroy = psock->saved_destroy;
 	sock_map_remove_links(sk, psock);
 	rcu_read_unlock();
-	sk_psock_stop(psock, false);
+	sk_psock_stop(psock);
 	sk_psock_put(sk, psock);
 	saved_destroy(sk);
 }
@@ -1619,9 +1619,10 @@ void sock_map_close(struct sock *sk, long timeout)
 	saved_close = psock->saved_close;
 	sock_map_remove_links(sk, psock);
 	rcu_read_unlock();
-	sk_psock_stop(psock, true);
-	sk_psock_put(sk, psock);
+	sk_psock_stop(psock);
 	release_sock(sk);
+	cancel_work_sync(&psock->work);
+	sk_psock_put(sk, psock);
 	saved_close(sk, timeout);
 }
 EXPORT_SYMBOL_GPL(sock_map_close);