linux-kernel - Re: [PATCH net-next v2 1/3] vsock: Linger on unsent data

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Message-ID: <km2nad6hkdi3ngtho2xexyhhosh4aq37scir2hgxkcfiwes2wd@5dyliiq7cpuh>
Date: Wed, 23 Apr 2025 17:53:12 +0200
From: Luigi Leonardi <leonardi@...hat.com>
To: Michal Luczaj <mhal@...x.co>
Cc: Stefano Garzarella <sgarzare@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Stefan Hajnoczi <stefanha@...hat.com>, virtualization@...ts.linux.dev, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/3] vsock: Linger on unsent data

Hi Michal,

On Mon, Apr 21, 2025 at 11:50:41PM +0200, Michal Luczaj wrote:
>Currently vsock's lingering effectively boils down to waiting (or timing
>out) until packets are consumed or dropped by the peer; be it by receiving
>the data, closing or shutting down the connection.
>
>To align with the semantics described in the SO_LINGER section of man
>socket(7) and to mimic AF_INET's behaviour more closely, change the logic
>of a lingering close(): instead of waiting for all data to be handled,
>block until data is considered sent from the vsock's transport point of
>view. That is until worker picks the packets for processing and decrements
>virtio_vsock_sock::bytes_unsent down to 0.
>
>Note that such lingering is limited to transports that actually implement
>vsock_transport::unsent_bytes() callback. This excludes Hyper-V and VMCI,
>under which no lingering would be observed.
>
>The implementation does not adhere strictly to man page's interpretation of
>SO_LINGER: shutdown() will not trigger the lingering. This follows AF_INET.
>
>Signed-off-by: Michal Luczaj <mhal@...x.co>
>---
> net/vmw_vsock/virtio_transport_common.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 7f7de6d8809655fe522749fbbc9025df71f071bd..aeb7f3794f7cfc251dde878cb44fdcc54814c89c 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -1196,12 +1196,21 @@ static void virtio_transport_wait_close(struct sock *sk, long timeout)
> {
> 	if (timeout) {
> 		DEFINE_WAIT_FUNC(wait, woken_wake_function);
>+		ssize_t (*unsent)(struct vsock_sock *vsk);
>+		struct vsock_sock *vsk = vsock_sk(sk);
>+
>+		/* Some transports (Hyper-V, VMCI) do not implement
>+		 * unsent_bytes. For those, no lingering on close().
>+		 */
>+		unsent = vsk->transport->unsent_bytes;
>+		if (!unsent)
>+			return;

IIUC if `unsent_bytes` is not implemented, virtio_transport_wait_close 
basically does nothing. My concern is that we are breaking the userspace 
due to a change in the behavior: Before this patch, with a vmci/hyper-v 
transport, this function would wait for SOCK_DONE to be set, but not 
anymore.

>
> 		add_wait_queue(sk_sleep(sk), &wait);
>
> 		do {
>-			if (sk_wait_event(sk, &timeout,
>-					  sock_flag(sk, SOCK_DONE), 
>&wait))
>+			if (sk_wait_event(sk, &timeout, unsent(vsk) == 
>0,
>+					  &wait))
> 				break;
> 		} while (!signal_pending(current) && timeout);
>
>
>-- 2.49.0
>

Thanks,
Luigi