Re: [PATCH v2] virtio-scsi: fix race in virtio_scsi_dataplane_start()

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

On Mon, Aug 08, 2022 at 12:21:34PM -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. This fix is taken from the virtio-blk
> dataplane code and it's worth adding a comment in virtio-blk as well to
> explain why it works.
> 
> 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>

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

> ---
> v2:
> - Switch to a smaller fix inspired by virtio-blk and document why
>   virtio-blk works [Paolo]
> 
>  hw/block/dataplane/virtio-blk.c |  5 +++++
>  hw/scsi/virtio-scsi-dataplane.c | 11 ++++++++---
>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 49276e46f2..26f965cabc 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -219,6 +219,11 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
>  
>      memory_region_transaction_commit();
>  
> +    /*
> +     * These fields are visible to the IOThread so we rely on implicit barriers
> +     * in aio_context_acquire() on the write side and aio_notify_accept() on
> +     * the read side.
> +     */
>      s->starting = false;
>      vblk->dataplane_started = true;
>      trace_virtio_blk_data_plane_start(s);
> diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
> index 8bb6e6acfc..20bb91766e 100644
> --- a/hw/scsi/virtio-scsi-dataplane.c
> +++ b/hw/scsi/virtio-scsi-dataplane.c
> @@ -136,6 +136,14 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
>  
>      memory_region_transaction_commit();
>  
> +    /*
> +     * These fields are visible to the IOThread so we rely on implicit barriers
> +     * in aio_context_acquire() on the write side and aio_notify_accept() on
> +     * the read side.
> +     */
> +    s->dataplane_starting = false;
> +    s->dataplane_started = true;
> +
>      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);
> @@ -143,9 +151,6 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
>      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;
>      aio_context_release(s->ctx);
>      return 0;
>  
> -- 
> 2.37.1