virtio/vhost-user: fix qemu abort when hotunplug vhost-user-net device

virtio/vhost-user: fix qemu abort when hotunplug vhost-user-net device

During the hot-unplugging of vhost-user-net type network cards,
the vhost_user_cleanup function may add the same rcu node to
the rcu linked list. The function call in this case is as follows:

vhost_user_cleanup
    ->vhost_user_host_notifier_remove
        ->call_rcu(n, vhost_user_host_notifier_free, rcu);
    ->g_free_rcu(n, rcu);

When this happens, QEMU will abort in try_dequeue:

if (head == &dummy && qatomic_mb_read(&tail) == &dummy.next) {
    abort();
}

backtrace is as follows:
0  __pthread_kill_implementation () at /usr/lib64/libc.so.6
1  raise () at /usr/lib64/libc.so.6
2  abort () at /usr/lib64/libc.so.6
3  try_dequeue () at ../util/rcu.c:235
4  call_rcu_thread (0) at ../util/rcu.c:288
5  qemu_thread_start (0) at ../util/qemu-thread-posix.c:541
6  start_thread () at /usr/lib64/libc.so.6
7  clone3 () at /usr/lib64/libc.so.6

The reason for the abort is that adding two identical nodes to
the rcu linked list will cause the rcu linked list to become a ring,
but when the dummy node is added after the two identical nodes,
the ring is opened. But only one node is added to list with
rcu_call_count added twice. This will cause rcu try_dequeue abort.

This happens when n->addr != 0. In some scenarios, this does happen.
For example, this situation will occur when using a 32-queue DPU
vhost-user-net type network card for hot-unplug testing, because
VhostUserHostNotifier->addr will be cleared during the processing of
VHOST_USER_BACKEND_VRING_HOST_NOTIFIER_MSG. However,it is asynchronous,
so we cannot guarantee that VhostUserHostNotifier->addr is zero in
vhost_user_cleanup. Therefore, it is necessary to merge g_free_rcu
and vhost_user_host_notifier_free into one rcu node.

Fixes: 503e355465 ("virtio/vhost-user: dynamically assign VhostUserHostNotifiers")
Signed-off-by: yaozhenguo <yaozhenguo@jd.com>
Message-Id: <20241011102913.45582-1-yaozhenguo@jd.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>