Re: [PATCH] virtio-scsi: fix race in virtio_scsi_dataplane_start()

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

On Wed, Aug 03, 2022 at 12:28:24PM -0400, Stefan Hajnoczi wrote:
> As soon as virtio_scsi_data_plane_start() attaches host notifiers the
> IOThread may start virtqueue processing. There is a race between
> IOThread virtqueue processing and virtio_scsi_data_plane_start() because
> it only assigns s->dataplane_started after attaching host notifiers.
> 
> When a virtqueue handler function in the IOThread calls
> virtio_scsi_defer_to_dataplane() it may see !s->dataplane_started and
> attempt to start dataplane even though we're already in the IOThread:
> 
>   #0  0x00007f67b360857c __pthread_kill_implementation (libc.so.6 + 0xa257c)
>   #1  0x00007f67b35bbd56 raise (libc.so.6 + 0x55d56)
>   #2  0x00007f67b358e833 abort (libc.so.6 + 0x28833)
>   #3  0x00007f67b358e75b __assert_fail_base.cold (libc.so.6 + 0x2875b)
>   #4  0x00007f67b35b4cd6 __assert_fail (libc.so.6 + 0x4ecd6)
>   #5  0x000055ca87fd411b memory_region_transaction_commit (qemu-kvm + 0x67511b)
>   #6  0x000055ca87e17811 virtio_pci_ioeventfd_assign (qemu-kvm + 0x4b8811)
>   #7  0x000055ca87e14836 virtio_bus_set_host_notifier (qemu-kvm + 0x4b5836)
>   #8  0x000055ca87f8e14e virtio_scsi_set_host_notifier (qemu-kvm + 0x62f14e)
>   #9  0x000055ca87f8dd62 virtio_scsi_dataplane_start (qemu-kvm + 0x62ed62)
>   #10 0x000055ca87e14610 virtio_bus_start_ioeventfd (qemu-kvm + 0x4b5610)
>   #11 0x000055ca87f8c29a virtio_scsi_handle_ctrl (qemu-kvm + 0x62d29a)
>   #12 0x000055ca87fa5902 virtio_queue_host_notifier_read (qemu-kvm + 0x646902)
>   #13 0x000055ca882c099e aio_dispatch_handler (qemu-kvm + 0x96199e)
>   #14 0x000055ca882c1761 aio_poll (qemu-kvm + 0x962761)
>   #15 0x000055ca880e1052 iothread_run (qemu-kvm + 0x782052)
>   #16 0x000055ca882c562a qemu_thread_start (qemu-kvm + 0x96662a)
> 
> This patch assigns s->dataplane_started before attaching host notifiers
> so that virtqueue handler functions that run in the IOThread before
> virtio_scsi_data_plane_start() returns correctly identify that dataplane
> does not need to be started.
> 
> Note that s->dataplane_started does not need the AioContext lock because
> it is set before attaching host notifiers and cleared after detaching
> host notifiers. In other words, the IOThread always sees the value true
> and the main loop thread does not modify it while the IOThread is
> active.
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2099541
> Reported-by: Qing Wang <qinwang@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

A scsi thing that tree seems more appropriate.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>



> ---
>  hw/scsi/virtio-scsi-dataplane.c | 33 +++++++++++++++++++++++++--------
>  1 file changed, 25 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
> index 8bb6e6acfc..a575c3f0cd 100644
> --- a/hw/scsi/virtio-scsi-dataplane.c
> +++ b/hw/scsi/virtio-scsi-dataplane.c
> @@ -66,6 +66,21 @@ static int virtio_scsi_set_host_notifier(VirtIOSCSI *s, VirtQueue *vq, int n)
>      return 0;
>  }
>  
> +/* Context: BH in IOThread */
> +static void virtio_scsi_dataplane_start_bh(void *opaque)
> +{
> +    VirtIOSCSI *s = opaque;
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
> +    int i;
> +
> +    virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);
> +    virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx);
> +
> +    for (i = 0; i < vs->conf.num_queues; i++) {
> +        virtio_queue_aio_attach_host_notifier(vs->cmd_vqs[i], s->ctx);
> +    }
> +}
> +
>  /* Context: BH in IOThread */
>  static void virtio_scsi_dataplane_stop_bh(void *opaque)
>  {
> @@ -136,16 +151,18 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
>  
>      memory_region_transaction_commit();
>  
> -    aio_context_acquire(s->ctx);
> -    virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);
> -    virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx);
> -
> -    for (i = 0; i < vs->conf.num_queues; i++) {
> -        virtio_queue_aio_attach_host_notifier(vs->cmd_vqs[i], s->ctx);
> -    }
> -
>      s->dataplane_starting = false;
>      s->dataplane_started = true;
> +
> +    /*
> +     * Attach notifiers from within the IOThread. It's possible to attach
> +     * notifiers from our thread directly but this approach has the advantages
> +     * that virtio_scsi_dataplane_start_bh() is symmetric with
> +     * virtio_scsi_dataplane_stop_bh() and the s->dataplane_started assignment
> +     * above doesn't require explicit synchronization.
> +     */
> +    aio_context_acquire(s->ctx);
> +    aio_wait_bh_oneshot(s->ctx, virtio_scsi_dataplane_start_bh, s);
>      aio_context_release(s->ctx);
>      return 0;
>  
> -- 
> 2.37.1