On Thu, Jun 29, 2023 at 02:54:15PM +0800, Rma Ma wrote:
> fix: QEMU deadlock with dpdk-vdpa
>
> QEMU start vhost-user with modern net and blk, backend use dpdk-vdpa process,
> after live migration, dest QEMU deadlock with dpdk-vdpa
>
> - QEMU sends VHOST_USER_SET_VRING_KICK to dpdk-vdpa net
> - QEMU does not need to wait for a response to this message
> - QEMU then sends VHOST_USER_SET_MEM_TABLE to dpdk-vdpa blk
> - QEMU needs to wait reply in this message
> - when dpdk-vdpa recv VHOST_USER_SET_VRING_KICK, it will send
> VHOST_USER_BACKEND_VRING_HOST_NOTIFIER_MSG to QEMU
> - dpdk-vdpa needs to wait for a response to this message
> - since QEMU vhost_user_read and backend channel are synchronous in the same
> thread
> - QEMU will deadlock with dpdk-vdpa
>
> Signed-off-by: Rma Ma <rma...@jaguarmicro.com>
Hmm this will need some thought. I'm concerned that this
is making what was previously a simple synchronous
code, multithreaded.
Also I feel dpdk needs to fix this too.
Let's document in the doc that when backend is allowed to send messages
(VHOST_USER_PROTOCOL_F_BACKEND_REQ) , neither side is allowed to block
processing incoming messsages while waiting for response.
> ---
> hw/virtio/vhost-user.c | 67 ++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 61 insertions(+), 6 deletions(-)
>
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index c4e0cbd702..1f6b3a5a63 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -274,6 +274,17 @@ struct scrub_regions {
> int fd_idx;
> };
>
> +struct backend_thread {
> + QemuThread thread;
> + QemuSemaphore init_done_sem;
> + int thread_id;
> + GMainContext *ctxt;
> + GMainLoop *loop;
> +};
> +
> +static struct backend_thread *backend_th;
> +static bool backend_thread_run;
> +
> static bool ioeventfd_enabled(void)
> {
> return !kvm_enabled() || kvm_eventfds_enabled();
> @@ -1696,7 +1707,8 @@ fdcleanup:
> return rc;
> }
>
> -static int vhost_setup_backend_channel(struct vhost_dev *dev)
> +static int vhost_setup_backend_channel(struct vhost_dev *dev,
> + GMainContext *ctxt)
> {
> VhostUserMsg msg = {
> .hdr.request = VHOST_USER_SET_BACKEND_REQ_FD,
> @@ -1728,7 +1740,7 @@ static int vhost_setup_backend_channel(struct vhost_dev
> *dev)
> u->backend_ioc = ioc;
> u->backend_src = qio_channel_add_watch_source(u->backend_ioc,
> G_IO_IN | G_IO_HUP,
> - backend_read, dev, NULL,
> NULL);
> + backend_read, dev, NULL,
> ctxt);
>
> if (reply_supported) {
> msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
> @@ -1981,6 +1993,42 @@ static int
> vhost_user_postcopy_notifier(NotifierWithReturn *notifier,
> return 0;
> }
>
> +static void *vhost_backend_channel_worker(void *opaque)
> +{
> + struct backend_thread *backend_th = opaque;
> +
> + rcu_register_thread();
> +
> + backend_th->ctxt = g_main_context_new();
> + backend_th->loop = g_main_loop_new(backend_th->ctxt, false);
> + backend_th->thread_id = qemu_get_thread_id();
> +
> + qemu_sem_post(&backend_th->init_done_sem);
> +
> + g_main_loop_run(backend_th->loop);
> +
> + g_main_loop_unref(backend_th->loop);
> + g_main_context_unref(backend_th->ctxt);
> + g_free(backend_th);
> + rcu_unregister_thread();
> + return NULL;
> +}
> +
> +static void vhost_backend_thread_init(void)
> +{
> + backend_th = g_malloc0(sizeof(struct backend_thread));
> + backend_th->thread_id = -1;
> + qemu_sem_init(&backend_th->init_done_sem, 0);
> +
> + qemu_thread_create(&backend_th->thread, "backend-channel-worker",
> + vhost_backend_channel_worker, backend_th,
> + QEMU_THREAD_DETACHED);
> +
> + while (backend_th->thread_id == -1) {
> + qemu_sem_wait(&backend_th->init_done_sem);
> + }
> +}
> +
> static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
> Error **errp)
> {
> @@ -2108,10 +2156,17 @@ static int vhost_user_backend_init(struct vhost_dev
> *dev, void *opaque,
> }
>
> if (dev->vq_index == 0) {
> - err = vhost_setup_backend_channel(dev);
> - if (err < 0) {
> - error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
> - return -EPROTO;
> + if (!backend_thread_run) {
> + vhost_backend_thread_init();
> + backend_thread_run = true;
> + }
> +
> + if (backend_thread_run && backend_th) {
> + err = vhost_setup_backend_channel(dev, backend_th->ctxt);
> + if (err < 0) {
> + error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
> + return -EPROTO;
> + }
> }
> }
>
> --
> 2.17.1
