Re: [PATCH net] vsock/bpf: release sock lock while waiting for data in recvmsg

[Stefano Garzarella]

+cc Bobby and Michal who touched this code recently

On Tue, Sep 29, 2026 at 06:33:04PM +0200, Jerome Mohm via B4 Relay wrote:
> From: Jerome Mohm <jrmmhm.kernel@xxxxxxxxx>
>
> vsock_bpf_recvmsg() takes lock_sock(sk) and holds it across the whole
> receive loop, including vsock_msg_wait_data(), which sleeps in
> wait_woken() without dropping the lock. When data arrives, the
> transport's delivery context (the vsock-loopback worker, or the
> virtio/vhost rx path) calls virtio_transport_recv_pkt() -> lock_sock()
> on the same socket and blocks. That context is the only one that would
> queue the skb and call sk_data_ready() to wake the reader, so neither
> side makes progress. A blocking recv() hangs until a signal arrives (a
> finite SO_RCVTIMEO also breaks it); while it lasts the shared delivery
> worker is stalled, so all vsock rx on that transport stops, not only
> the affected socket. The hung-task watchdog reports the worker blocked
> in D state:
>
>  INFO: task kworker/1:3:107 blocked for more than 5 seconds.
>  task:kworker/1:3 state:D Workqueue: vsock-loopback vsock_loopback_work
>  Call Trace:
>   __schedule
>   schedule
>   __lock_sock
>   lock_sock_nested
>   virtio_transport_recv_pkt
>   vsock_loopback_work
>
> The reader holds the same sk_lock-AF_VSOCK it is waiting on, taken in
> vsock_bpf_recvmsg().
>
> This code is based on net/unix/unix_bpf.c, whose unix_msg_wait_data()
> drops u->iolock around wait_woken() and re-takes it afterwards. The
> vsock port substituted lock_sock() for that serialisation lock but
> omitted the unlock and relock. tcp_bpf and the native
> vsock_connectible_wait_data() both drop the lock across the wait;
> vsock_bpf is the only one that does not.
>
> Release the socket lock around the wait and re-acquire it before
> re-checking for data, so the caller's locking is unchanged.
>
> Fixes: 634f1a7110b4 ("vsock: support sockmap")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Jerome Mohm <jrmmhm.kernel@xxxxxxxxx>
> ---
> net/vmw_vsock/vsock_bpf.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/net/vmw_vsock/vsock_bpf.c b/net/vmw_vsock/vsock_bpf.c
> index 9049d2648646..bb7d81a95baa 100644
> --- a/net/vmw_vsock/vsock_bpf.c
> +++ b/net/vmw_vsock/vsock_bpf.c
> @@ -50,7 +50,9 @@ static bool vsock_msg_wait_data(struct sock *sk, struct sk_psock *psock, long ti
> 	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> 	ret = vsock_has_data(sk, psock);
> 	if (!ret) {
> +		release_sock(sk);
> 		wait_woken(&wait, TASK_INTERRUPTIBLE, timeo);
> +		lock_sock(sk);
> 		ret = vsock_has_data(sk, psock);
> 	}
> 	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>
> ---
> base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
> change-id: 20260929-kbh3-1-022-fix-340aa3f7f071
>
> Best regards,
> --
> Jerome Mohm <jrmmhm.kernel@xxxxxxxxx>

LGTM, but I'd like also Bobby and Michal opinion:

Acked-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>

Thanks,
Stefano