Re: [PATCH v3 1/2] vhost-user: fix VirtQ notifier cleanup

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 08, 2021 at 03:58:04PM +0800, Xueming Li wrote:
> When vhost-user device cleanup and unmmap notifier address, VM cpu
> thread that writing the notifier failed with accessing invalid address.
> 
> To avoid this concurrent issue, wait memory flatview update by draining
> rcu callbacks, then unmap notifiers.
> 
> Fixes: 44866521bd6e ("vhost-user: support registering external host notifiers")
> Cc: tiwei.bie@intel.com
> Cc: qemu-stable@nongnu.org
> Cc: Yuwei Zhang <zhangyuwei.9149@bytedance.com>
> Signed-off-by: Xueming Li <xuemingl@nvidia.com>
> ---
>  hw/virtio/vhost-user.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index bf6e50223c..b2e948bdc7 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1165,6 +1165,12 @@ static void vhost_user_host_notifier_remove(struct vhost_dev *dev,
>  
>      if (n->addr && n->set) {
>          virtio_queue_set_host_notifier_mr(vdev, queue_idx, &n->mr, false);
> +        if (!qemu_in_vcpu_thread()) { /* Avoid vCPU dead lock. */
> +            /* Wait for VM threads accessing old flatview which contains notifier. */
> +            drain_call_rcu();
> +        }
> +        munmap(n->addr, qemu_real_host_page_size);
> +        n->addr = NULL;
>          n->set = false;
>      }
>  }


../hw/virtio/vhost-user.c: In function ‘vhost_user_host_notifier_remove’:
../hw/virtio/vhost-user.c:1168:14: error: implicit declaration of function ‘qemu_in_vcpu_thread’ [-Werror=implicit-function-declaration]
 1168 |         if (!qemu_in_vcpu_thread()) { /* Avoid vCPU dead lock. */
      |              ^~~~~~~~~~~~~~~~~~~
../hw/virtio/vhost-user.c:1168:14: error: nested extern declaration of ‘qemu_in_vcpu_thread’ [-Werror=nested-externs]
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.
make[1]: *** [Makefile:162: run-ninja] Error 1
make[1]: Leaving directory '/scm/qemu/build'
make: *** [GNUmakefile:11: all] Error 2


Although the following patch fixes it, bisect is broken.


> @@ -1502,12 +1508,7 @@ static int vhost_user_slave_handle_vring_host_notifier(struct vhost_dev *dev,
>  
>      n = &user->notifier[queue_idx];
>  
> -    if (n->addr) {
> -        virtio_queue_set_host_notifier_mr(vdev, queue_idx, &n->mr, false);
> -        object_unparent(OBJECT(&n->mr));
> -        munmap(n->addr, page_size);
> -        n->addr = NULL;
> -    }
> +    vhost_user_host_notifier_remove(dev, queue_idx);
>  
>      if (area->u64 & VHOST_USER_VRING_NOFD_MASK) {
>          return 0;
> @@ -2485,11 +2486,17 @@ void vhost_user_cleanup(VhostUserState *user)
>      for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
>          if (user->notifier[i].addr) {
>              object_unparent(OBJECT(&user->notifier[i].mr));
> +        }
> +    }
> +    memory_region_transaction_commit();
> +    /* Wait for VM threads accessing old flatview which contains notifier. */
> +    drain_call_rcu();
> +    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
> +        if (user->notifier[i].addr) {
>              munmap(user->notifier[i].addr, qemu_real_host_page_size);
>              user->notifier[i].addr = NULL;
>          }
>      }
> -    memory_region_transaction_commit();
>      user->chr = NULL;
>  }
>  
> -- 
> 2.33.0