Re: [Qemu-devel] [PATCH 06/11] block: add bdrv_drain()

[Max Reitz <mreitz@redhat.com>]

On 01.10.2014 19:01, Stefan Hajnoczi wrote:
> Now that op blockers are in use, we can ensure that no other sources are
> generating I/O on a BlockDriverState.  Therefore it is possible to drain
> requests for a single BDS.
>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>   block.c               | 36 +++++++++++++++++++++++++++++-------
>   include/block/block.h |  1 +
>   2 files changed, 30 insertions(+), 7 deletions(-)
>
> diff --git a/block.c b/block.c
> index c5a251c..48305c4 100644
> --- a/block.c
> +++ b/block.c
> @@ -1918,6 +1918,34 @@ static bool bdrv_requests_pending(BlockDriverState *bs)
>       return false;
>   }
>   
> +static bool bdrv_drain_one(BlockDriverState *bs)
> +{
> +    bool bs_busy;
> +
> +    bdrv_flush_io_queue(bs);
> +    bdrv_start_throttled_reqs(bs);
> +    bs_busy = bdrv_requests_pending(bs);
> +    bs_busy |= aio_poll(bdrv_get_aio_context(bs), bs_busy);
> +    return bs_busy;
> +}
> +
> +/*
> + * Wait for pending requests to complete on a single BlockDriverState subtree
> + *
> + * See the warning in bdrv_drain_all().  This function can only be called if
> + * you are sure nothing can generate I/O because you have op blockers
> + * installed.

Although that warning now sounds too harsh: "it is not possible to have 
a function to drain a single device's I/O queue." Apparently, under 
certain circumstances, it now *is* possible. ;-)

Reviewed-by: Max Reitz <mreitz@redhat.com>