[PATCH net-next v2] vsock: avoid timeout for non-blocking accept() with empty backlog

[PATCH net-next v2] vsock: avoid timeout for non-blocking accept() with empty backlog

[Laurence Rowe]

A common pattern in epoll network servers is to eagerly accept all
pending connections from the non-blocking listening socket after
epoll_wait indicates the socket is ready by calling accept in a loop
until EAGAIN is returned indicating that the backlog is empty.

Scheduling a timeout for a non-blocking accept with an empty backlog
meant AF_VSOCK sockets used by epoll network servers incurred hundreds
of microseconds of additional latency per accept loop compared to
AF_INET or AF_UNIX sockets.

Signed-off-by: Laurence Rowe <laurencerowe@gmail.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
---

Patch v2 with feedback from Stefano Garzarella and Bobby Eshleman:

- Move prepare_to_wait before release_sock to match previous behaviour.

- Simplify not connected case.

This fixes the observed issue for me:

1. With loopback vsock on the host running Linux v6.19.10 built with
config-6.17.0-19-generic from Ubuntu 24.04 and make olddefconfig.

2. With Firecracker guests with current torvalds/master, v6.19.10, and
amazonlinux/microvm-kernel-6.1.166-24.303.amzn2023 used in Firecracker
CI and examples. (Firecracker guest vsocks are unix sockets on the host
side so this fix works there with just a fixed guest kernel.)

I struggled to build a generic 6.1.166 kernel that worked as a
Firecracker guest but the patch applies (conflict due to change of
`flags` to `arg->flags` in surrounding context) so I believe it should
work for generic v6.1.166 kernel.

Alternatively a minimal version of this fix is to just wrap the
`schedule_timeout` in an `if (timeout != 0)` but that leaves an
unnecessary additional `lock_sock` call.

There are ftrace's and reproduction tools at:
https://github.com/lrowe/linux-vsock-accept-timeout-investigation
---
 net/vmw_vsock/af_vsock.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index e4756604d5..b8794ea0f0 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -1864,10 +1864,10 @@ static int vsock_accept(struct socket *sock, struct socket *newsock,
 	 * created upon connection establishment.
 	 */
 	timeout = sock_rcvtimeo(listener, arg->flags & O_NONBLOCK);
-	prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE);
 
 	while ((connected = vsock_dequeue_accept(listener)) == NULL &&
-	       listener->sk_err == 0) {
+	       listener->sk_err == 0 && timeout != 0) {
+		prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE);
 		release_sock(listener);
 		timeout = schedule_timeout(timeout);
 		finish_wait(sk_sleep(listener), &wait);
@@ -1876,17 +1876,14 @@ static int vsock_accept(struct socket *sock, struct socket *newsock,
 		if (signal_pending(current)) {
 			err = sock_intr_errno(timeout);
 			goto out;
-		} else if (timeout == 0) {
-			err = -EAGAIN;
-			goto out;
 		}
-
-		prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE);
 	}
-	finish_wait(sk_sleep(listener), &wait);
 
-	if (listener->sk_err)
+	if (listener->sk_err) {
 		err = -listener->sk_err;
+	} else if (!connected) {
+		err = -EAGAIN;
+	}
 
 	if (connected) {
 		sk_acceptq_removed(listener);

base-commit: f35340f2d653f1003602878403c901396ab03c17
-- 
2.43.0

Re: [PATCH net-next v2] vsock: avoid timeout for non-blocking accept() with empty backlog

[Stefano Garzarella]

On Thu, Apr 02, 2026 at 01:49:18PM -0700, Laurence Rowe wrote:
>A common pattern in epoll network servers is to eagerly accept all
>pending connections from the non-blocking listening socket after
>epoll_wait indicates the socket is ready by calling accept in a loop
>until EAGAIN is returned indicating that the backlog is empty.
>
>Scheduling a timeout for a non-blocking accept with an empty backlog
>meant AF_VSOCK sockets used by epoll network servers incurred hundreds
>of microseconds of additional latency per accept loop compared to
>AF_INET or AF_UNIX sockets.
>
>Signed-off-by: Laurence Rowe <laurencerowe@gmail.com>
>Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
>---
>
>Patch v2 with feedback from Stefano Garzarella and Bobby Eshleman:
>
>- Move prepare_to_wait before release_sock to match previous behaviour.
>
>- Simplify not connected case.

Thanks for addressing them, now LGTM!

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

Re: [PATCH net-next v2] vsock: avoid timeout for non-blocking accept() with empty backlog

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu,  2 Apr 2026 13:49:18 -0700 you wrote:
> A common pattern in epoll network servers is to eagerly accept all
> pending connections from the non-blocking listening socket after
> epoll_wait indicates the socket is ready by calling accept in a loop
> until EAGAIN is returned indicating that the backlog is empty.
> 
> Scheduling a timeout for a non-blocking accept with an empty backlog
> meant AF_VSOCK sockets used by epoll network servers incurred hundreds
> of microseconds of additional latency per accept loop compared to
> AF_INET or AF_UNIX sockets.
> 
> [...]

Here is the summary with links:
  - [net-next,v2] vsock: avoid timeout for non-blocking accept() with empty backlog
    https://git.kernel.org/netdev/net-next/c/98f28d8d6e5a

You are awesome, thank you!
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