Re: [Qemu-devel] [PATCH 8/9] block: Introduce BlockDriver.bdrv_drain callback

[Kevin Wolf <kwolf@redhat.com>]

Am 26.10.2015 um 07:24 hat Fam Zheng geschrieben:
> Drivers can have internal request sources that generate IO, like the
> need_check_timer in QED. Since we want quiesced periods that contain
> nested event loops in block layer, we need to have a way to disable such
> event sources.
> 
> Block drivers must implement the "bdrv_drain" callback if it has any
> internal sources that can generate I/O activity, like a timer or a
> worker thread (even in a library) that can schedule QEMUBH in an
> asynchronous callback.
> 
> Update the comments of bdrv_drain and bdrv_drained_begin accordingly.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> Reviewed-by: Jeff Cody <jcody@redhat.com>
> Reviewed-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/io.c                | 6 +++++-
>  include/block/block_int.h | 6 ++++++
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 358c3c4..a740827 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -234,7 +234,8 @@ bool bdrv_requests_pending(BlockDriverState *bs)
>  }
>  
>  /*
> - * Wait for pending requests to complete on a single BlockDriverState subtree
> + * Wait for pending requests to complete on a single BlockDriverState subtree,
> + * and suspend block driver's internal I/O until next request arrives.
>   *
>   * Note that unlike bdrv_drain_all(), the caller must hold the BlockDriverState
>   * AioContext.
> @@ -247,6 +248,9 @@ void bdrv_drain(BlockDriverState *bs)
>  {
>      bool busy = true;
>  
> +    if (bs->drv && bs->drv->bdrv_drain) {
> +        bs->drv->bdrv_drain(bs);
> +    }

Does this need to be recursive? Recursing into bs->file and bs->backing
would be consistent with the current bdrv_requests_pending(); but I'm
planning to send a patch to extend this to all children.

Kevin