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

[Stefano Garzarella <sgarzare@redhat.com>]

+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@eldare.de>
>
>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@vger.kernel.org
>Assisted-by: LLM
>Signed-off-by: Jerome Mohm <jrmmhm.kernel@eldare.de>
>---
> 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@eldare.de>
>

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

Acked-by: Stefano Garzarella <sgarzare@redhat.com>

Thanks,
Stefano