Subject: Re: [Patch bpf] sock_map: convert cancel_work_sync() to cancel_work()

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 24 Oct 2022 15:33:13 +0200

On Tue, Oct 18, 2022 at 11:13 AM -07, [email-masked] wrote:
> On 10/17, Cong Wang wrote:
>> From: Cong Wang <[email-masked]>
>
>> Technically we don't need lock the sock in the psock work, but we
>> need to prevent this work running in parallel with sock_map_close().
>
>> With this, we no longer need to wait for the psock->work synchronously,
>> because when we reach here, either this work is still pending, or
>> blocking on the lock_sock(), or it is completed. We only need to cancel
>> the first case asynchronously, and we need to bail out the second case
>> quickly by checking SK_PSOCK_TX_ENABLED bit.
>
>> Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
>> Reported-by: Stanislav Fomichev <[email-masked]>
>> Cc: John Fastabend <[email-masked]>
>> Cc: Jakub Sitnicki <[email-masked]>
>> Signed-off-by: Cong Wang <[email-masked]>
>
> This seems to remove the splat for me:
>
> Tested-by: Stanislav Fomichev <[email-masked]>
>
> The patch looks good, but I'll leave the review to Jakub/John.

I can't poke any holes in it either.

However, it is harder for me to follow than the initial idea [1].
So I'm wondering if there was anything wrong with it?

This seems like a step back when comes to simplifying locking in
sk_psock_backlog() that was done in 799aa7f98d53.

[1] https://lore.kernel.org/bpf/[email-masked]/T/#md486941e228a1b29729dba842ccd396c2c07d9fd

>
>> ---
>>   include/linux/skmsg.h |  2 +-
>>   net/core/skmsg.c      | 19 +++++++++++++------
>>   net/core/sock_map.c   |  4 ++--
>>   3 files changed, 16 insertions(+), 9 deletions(-)
>
>> 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..c329e71ea924 100644
>> --- a/net/core/skmsg.c
>> +++ b/net/core/skmsg.c
>> @@ -647,6 +647,11 @@ static void sk_psock_backlog(struct work_struct  *work)
>>   	int ret;
>
>>   	mutex_lock(&psock->work_mutex);
>> +	lock_sock(psock->sk);
>> +
>> +	if (!sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED))
>> +		goto end;
>> +
>>   	if (unlikely(state->skb)) {
>>   		spin_lock_bh(&psock->ingress_lock);
>>   		skb = state->skb;
>> @@ -672,9 +677,12 @@ static void sk_psock_backlog(struct work_struct  *work)
>>   		skb_bpf_redirect_clear(skb);
>>   		do {
>>   			ret = -EIO;
>> -			if (!sock_flag(psock->sk, SOCK_DEAD))
>> +			if (!sock_flag(psock->sk, SOCK_DEAD)) {
>> +				release_sock(psock->sk);
>>   				ret = sk_psock_handle_skb(psock, skb, off,
>>   							  len, ingress);
>> +				lock_sock(psock->sk);
>> +			}
>>   			if (ret <= 0) {
>>   				if (ret == -EAGAIN) {
>>   					sk_psock_skb_state(psock, state, skb,
>> @@ -695,6 +703,7 @@ static void sk_psock_backlog(struct work_struct *work)
>>   			kfree_skb(skb);
>>   	}
>>   end:
>> +	release_sock(psock->sk);
>>   	mutex_unlock(&psock->work_mutex);
>>   }
>
>> @@ -803,16 +812,14 @@ 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);
>> +	cancel_work(&psock->work);
>>   }
>
>>   static void sk_psock_done_strp(struct sk_psock *psock);
>> @@ -850,7 +857,7 @@ 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..d4e11d7f459c 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);
>>   }
>> @@ -1619,7 +1619,7 @@ 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_stop(psock);
>>   	sk_psock_put(sk, psock);
>>   	release_sock(sk);
>>   	saved_close(sk, timeout);
>> --
>> 2.34.1