On Tue, 1 Oct 2019, Florian Westphal wrote:

> tcp_poll places current task on a wait queue.  This can cause us to block
> forever -- some subflows might never receive any data because they're
> used as a backup/fallback path.
>
> This makes mptcp_poll wait on the mptcp socket wait queue instead.
> __tcp_poll is used instead to avoid blocking current task again after
> parent mptcp socket has been woken from the subflow layer.
>
> The series is incomplete, mptcp_should_wake_parent() should be extended
> to take packets off the subflow queue and add them to the mptcp socket
> queue, i.e. move most of the work currently done at recvmsg time into
> the wakeup function.
>
> This is also needed to not wake userspace in case the subflow tcp data
> was stale (e.g. because of too early mptcp-level retransmit).
>
> Signed-off-by: Florian Westphal <fw(a)strlen.de>
> ---
> net/mptcp/protocol.c |  9 ++++++++-
> net/mptcp/protocol.h |  1 +
> net/mptcp/subflow.c  | 12 +++++++++---
> 3 files changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index b54fecf528b9..908146b7f300 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -531,6 +531,11 @@ static enum mapping_status mptcp_get_mapping(struct sock *ssk)
> 	return ret;
> }
>
> +bool mptcp_should_wake_parent(struct mptcp_subflow_context *subflow)
> +{
> +	return true;
> +}
> +
> static void mptcp_wait_data(struct sock *sk, long *timeo)
> {
> 	DEFINE_WAIT_FUNC(wait, woken_wake_function);
> @@ -1378,11 +1383,13 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
> 		return ret;
> 	}
>
> +	sock_poll_wait(file, sock, wait);
> +
> 	mptcp_for_each_subflow(msk, subflow) {
> 		struct socket *tcp_sock;
>
> 		tcp_sock = mptcp_subflow_tcp_socket(subflow);
> -		ret |= tcp_poll(file, tcp_sock, wait);
> +		ret |= __tcp_poll(tcp_sock->sk);
> 	}
> 	release_sock(sk);
>
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index 0c6bc8617cf4..eaa48fe90629 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -265,6 +265,7 @@ void mptcp_get_options(const struct sk_buff *skb,
> void mptcp_finish_connect(struct sock *sk, int mp_capable);
> void mptcp_finish_join(struct sock *sk);
> void mptcp_reset_timer(struct sock *sk);
> +bool mptcp_should_wake_parent(struct mptcp_subflow_context *ctx);
>
> int mptcp_token_new_request(struct request_sock *req);
> void mptcp_token_destroy_request(u32 token);
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index fdec4f6208b7..2db2529ad323 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -310,14 +310,20 @@ static void subflow_data_ready(struct sock *sk)
> 	subflow->tcp_sk_data_ready(sk);
>
> 	if (parent) {
> -		pr_debug("parent=%p", parent);
> +		struct socket_wq *wq;
>
> 		smp_mb__before_atomic();
> 		set_bit(MPTCP_DATA_READY, &mptcp_sk(parent)->flags);
> 		smp_mb__after_atomic();
>
> -		parent->sk_data_ready(parent);
> -	}
> +		rcu_read_lock();
> +		wq = rcu_dereference(parent->sk_wq);
> +		if (skwq_has_sleeper(wq) &&
> +		    mptcp_should_wake_parent(subflow)) {
> +			wake_up_interruptible_all(&wq->wait);
> +		}
> +		rcu_read_unlock();

Compared to sock_def_readable, this wakeup code doesn't call 
sk_wake_async. Is SOCK_FASYNC irrelevant here (sk_wake_async doesn't do 
anything without that sock flag)?

Mat


> +	}
> }
>
> static void subflow_write_space(struct sock *sk)
> -- 
> 2.21.0
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>

--
Mat Martineau
Intel