Subject: Re: Lockdep warning after c0feea594e058223973db94c1c32a830c9807c86

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 24 Oct 2022 11:36:47 +0200

On Sun, Oct 16, 2022 at 11:11 AM -07, Cong Wang wrote:
> 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, [email-masked] 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 <[email-masked]>
>> 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/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);

Sorry for the delay. I've been out.

Great idea. I don't see why not.