Re: [Qemu-devel] [RFC PATCH] virtio-blk: schedule virtio_notify_config to run on main context

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

On Thu, Sep 12, 2019 at 08:19:25PM +0200, Sergio Lopez wrote:
> Another AioContext-related issue, and this is a tricky one.
> 
> Executing a QMP block_resize request for a virtio-blk device running
> on an iothread may cause a deadlock involving the following mutexes:
> 
>  - main thead
>   * Has acquired: qemu_mutex_global.
>   * Is trying the acquire: iothread AioContext lock via
>     AIO_WAIT_WHILE (after aio_poll).
> 
>  - iothread
>   * Has acquired: AioContext lock.
>   * Is trying to acquire: qemu_mutex_global (via
>     virtio_notify_config->prepare_mmio_access).

Hmm is this really the only case iothread takes qemu mutex?
If any such access can deadlock, don't we need a generic
solution? Maybe main thread can drop qemu mutex
before taking io thread AioContext lock?

> With this change, virtio_blk_resize checks if it's being called from a
> coroutine context running on a non-main thread, and if that's the
> case, creates a new coroutine and schedules it to be run on the main
> thread.
> 
> This works, but means the actual operation is done
> asynchronously, perhaps opening a window in which a "device_del"
> operation may fit and remove the VirtIODevice before
> virtio_notify_config() is executed.
> 
> I *think* it shouldn't be possible, as BHs will be processed before
> any new QMP/monitor command, but I'm open to a different approach.
> 
> RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1744955
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  hw/block/virtio-blk.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 18851601cb..c763d071f6 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -16,6 +16,7 @@
>  #include "qemu/iov.h"
>  #include "qemu/module.h"
>  #include "qemu/error-report.h"
> +#include "qemu/main-loop.h"
>  #include "trace.h"
>  #include "hw/block/block.h"
>  #include "hw/qdev-properties.h"
> @@ -1086,11 +1087,33 @@ static int virtio_blk_load_device(VirtIODevice *vdev, QEMUFile *f,
>      return 0;
>  }
>  
> +static void coroutine_fn virtio_resize_co_entry(void *opaque)
> +{
> +    VirtIODevice *vdev = opaque;
> +
> +    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
> +    virtio_notify_config(vdev);
> +    aio_wait_kick();
> +}
> +
>  static void virtio_blk_resize(void *opaque)
>  {
>      VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
> +    Coroutine *co;
>  
> -    virtio_notify_config(vdev);
> +    if (qemu_in_coroutine() &&
> +        qemu_get_current_aio_context() != qemu_get_aio_context()) {
> +        /*
> +         * virtio_notify_config() needs to acquire the global mutex,
> +         * so calling it from a coroutine running on a non-main context
> +         * may cause a deadlock. Instead, create a new coroutine and
> +         * schedule it to be run on the main thread.
> +         */
> +        co = qemu_coroutine_create(virtio_resize_co_entry, vdev);
> +        aio_co_schedule(qemu_get_aio_context(), co);
> +    } else {
> +        virtio_notify_config(vdev);
> +    }
>  }
>  
>  static const BlockDevOps virtio_block_ops = {
> -- 
> 2.21.0