Re: [Qemu-devel] [PATCH] Revert "qed: Implement .bdrv_drain"

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/03/2016 11:11, Stefan Hajnoczi wrote:
> This reverts commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01.
> 
> Note that commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01 included some
> unrelated hunks, possibly due to a merge failure or an overlooked
> squash.  This only reverts the qed .bdrv_drain() implementation.
> 
> The qed .bdrv_drain() implementation is unsafe and can lead to a double
> request completion.
> 
> Paolo Bonzini reports:
> "The problem is that bdrv_qed_drain calls qed_plug_allocating_write_reqs
> unconditionally, but this is not correct if an allocating write is
> queued.  In this case, qed_unplug_allocating_write_reqs will restart the
> allocating write and possibly cause it to complete.  The aiocb however
> is still in use for the L2/L1 table writes, and will then be completed
> again as soon as the table writes are stable."
> 
> For QEMU 2.6 we can simply revert this commit.  A full solution for the
> qed need check timer may be added if the bdrv_drain() implementation is
> extended.
> 
> Reported-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo

> ---
>  block/qed.c | 13 -------------
>  1 file changed, 13 deletions(-)
> 
> Passes ./check -qed.
> 
> diff --git a/block/qed.c b/block/qed.c
> index 404be1e..3da8021 100644
> --- a/block/qed.c
> +++ b/block/qed.c
> @@ -376,18 +376,6 @@ static void bdrv_qed_attach_aio_context(BlockDriverState *bs,
>      }
>  }
>  
> -static void bdrv_qed_drain(BlockDriverState *bs)
> -{
> -    BDRVQEDState *s = bs->opaque;
> -
> -    /* Cancel timer and start doing I/O that were meant to happen as if it
> -     * fired, that way we get bdrv_drain() taking care of the ongoing requests
> -     * correctly. */
> -    qed_cancel_need_check_timer(s);
> -    qed_plug_allocating_write_reqs(s);
> -    bdrv_aio_flush(s->bs, qed_clear_need_check, s);
> -}
> -
>  static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
>                           Error **errp)
>  {
> @@ -1692,7 +1680,6 @@ static BlockDriver bdrv_qed = {
>      .bdrv_check               = bdrv_qed_check,
>      .bdrv_detach_aio_context  = bdrv_qed_detach_aio_context,
>      .bdrv_attach_aio_context  = bdrv_qed_attach_aio_context,
> -    .bdrv_drain               = bdrv_qed_drain,
>  };
>  
>  static void bdrv_qed_init(void)
>