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

[Laurence Rowe <laurencerowe@gmail.com>]

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>
---

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 | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 2f7d94d682..483889b6d8 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -1850,11 +1850,11 @@ 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) {
 		release_sock(listener);
+		prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE);
 		timeout = schedule_timeout(timeout);
 		finish_wait(sk_sleep(listener), &wait);
 		lock_sock(listener);
@@ -1862,17 +1862,15 @@ 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 (timeout == 0 && connected == NULL) {
+		err = -EAGAIN;
+		goto out;
+	}
 
 	if (connected) {
 		sk_acceptq_removed(listener);
-- 
2.43.0