Re: [Qemu-devel] [Qemu-block] [PATCH 2/3] virtio-blk: Use blk_drain() to drain IO requests

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Jun 10, 2015 at 02:38:16PM +0300, Alexander Yarygin wrote:
> Each call of the virtio_blk_reset() function calls blk_drain_all(),
> which works for all existing BlockDriverStates, while draining only
> one is needed.
> 
> This patch replaces blk_drain_all() by blk_drain() in virtio_blk_reset().
> 
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
> ---
>  hw/block/virtio-blk.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index e6afe97..2009092 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -652,15 +652,16 @@ static void virtio_blk_reset(VirtIODevice *vdev)
>  {
>      VirtIOBlock *s = VIRTIO_BLK(vdev);
>  
> -    if (s->dataplane) {
> -        virtio_blk_data_plane_stop(s->dataplane);
> -    }
> -
>      /*
>       * This should cancel pending requests, but can't do nicely until there
>       * are per-device request lists.
>       */
> -    blk_drain_all();
> +    blk_drain(s->blk);
> +
> +    if (s->dataplane) {
> +        virtio_blk_data_plane_stop(s->dataplane);
> +    }

This is unsafe now.  virtio_blk_reset() is called from the vcpu thread
while the virtqueue processing may be running in an IOThread.
blk_drain() does not acquire AioContext so it races with the IOThread.

Try:

ctx = bdrv_get_aio_context(s->blk);
aio_context_acquire(ctx);

blk_drain(s->blk);

if (s->dataplane) {
    virtio_blk_data_plane_stop(s->dataplane);
}

aio_context_release(ctx);

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