From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konstantin Khorenko Date: Tue, 18 Aug 2026 18:56:54 +0200 Subject: Re: [Devel] [PATCH vz10 4/5] fixup! vhost/vsock: re-scan TX virtqueue on device start In-Reply-To: <20260625181637.1555685-4-eva.kurchatova@virtuozzo.com> References: <20260625181637.1555685-1-eva.kurchatova@virtuozzo.com> <20260625181637.1555685-4-eva.kurchatova@virtuozzo.com> Message-ID: <10886452-9faa-493d-a26e-6f718ea76e4a@virtuozzo.com> List-Id: already applied here The commit is pushed to "branch-rh10-6.12.0-211.16.1.12.x.vz10-ovz" and will appear at git at bitbucket.org:openvz/vzkernel.git after rh10-6.12.0-211.16.1.12.5.vz10 ------> commit 67fdd1befc4da39f114de9a2ad4e725ea442a034 Author: Andrey Drobyshev Date: Thu Jun 4 18:46:13 2026 +0300 vhost/vsock: re-scan TX virtqueue on device start During QEMU CPR live-update (and VHOST_RESET_OWNER in general) the guest keeps running while the host drops and later re-attaches vhost backends. If the guest adds a buffer to the TX virtqueue (guest->host) and kicks while the backend is temporarily NULL (between vhost_vsock_drop_backends() and the next vhost_vsock_start()), then the kick is delivered to the vhost worker, handle_tx_kick() sees a NULL backend and returns, and the kick signal is consumed. The buffer is then left in the ring. Then upon device start vhost_vsock_start() only re-kicks the RX send worker, never the TX VQ, so the buffer is processed only if the guest happens to kick again. But if the guest itself is now waiting for data from the host, it will never kick TX VQ again, and we end up in a deadlock. The deadlock is reproduced during active host->guest socat data transfer under multiple consecutive qemu-update's. To fix this, in vhost_vsock_start(), after kicking the RX send worker, also queue the TX vq poll so any buffers the guest enqueued while we were paused get scanned. https://virtuozzo.atlassian.net/browse/VSTOR-131956 https://virtuozzo.atlassian.net/browse/VSTOR-101116 Reviewed-by: Pavel Tikhomirov Signed-off-by: Andrey Drobyshev Feature: vhost-vsock: VHOST_RESET_OWNER ioctl -- Best regards, Konstantin Khorenko, Virtuozzo Linux Kernel Team On 6/25/26 20:16, Eva Kurchatova wrote: > From: Andrey Drobyshev > > During QEMU CPR live-update (and VHOST_RESET_OWNER in general) the guest > keeps running while the host drops and later re-attaches vhost backends. > If the guest adds a buffer to the TX virtqueue (guest->host) and kicks > while the backend is temporarily NULL (between vhost_vsock_drop_backends() > and the next vhost_vsock_start()), then the kick is delivered to the > vhost worker, handle_tx_kick() sees a NULL backend and returns, and the > kick signal is consumed. The buffer is then left in the ring. > > Then upon device start vhost_vsock_start() only re-kicks the RX send > worker, never the TX VQ, so the buffer is processed only if the guest > happens to kick again. But if the guest itself is now waiting for data > from the host, it will never kick TX VQ again, and we end up in a > deadlock. > > The deadlock is reproduced during active host->guest socat data transfer > under multiple consecutive qemu-update's. > > To fix this, in vhost_vsock_start(), after kicking the RX send worker, also > queue the TX vq poll so any buffers the guest enqueued while we were paused > get scanned. > > https://virtuozzo.atlassian.net/browse/VSTOR-131956 > https://virtuozzo.atlassian.net/browse/VSTOR-101116 > > Signed-off-by: Andrey Drobyshev > > Feature: vhost-vsock: VHOST_RESET_OWNER ioctl > --- > drivers/vhost/vsock.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c > index f652f47956d7..2cee40a1ca04 100644 > --- a/drivers/vhost/vsock.c > +++ b/drivers/vhost/vsock.c > @@ -634,6 +634,12 @@ static int vhost_vsock_start(struct vhost_vsock *vsock) > /* See vhost_transport_send_pkt(); never cleared. */ > WRITE_ONCE(vsock->started_once, true); > > + /* > + * Some packets might've also been queued in TX VQ. Re-scan it here, > + * mirroring the RX send-worker kick above. > + */ > + vhost_poll_queue(&vsock->vqs[VSOCK_VQ_TX].poll); > + > mutex_unlock(&vsock->dev.mutex); > return 0; >