Re: [Qemu-devel] [Qemu-block] [PATCH v5 11/12] qed: Implement .bdrv_drain

[Jeff Cody <jcody@redhat.com>]

On Wed, Oct 21, 2015 at 10:06:48AM +0800, Fam Zheng wrote:
> The "need_check_timer" is used to clear the "NEED_CHECK" flag in the
> image header after a grace period once metadata update has finished. In
> compliance to the bdrv_drain semantics we should make sure it remains
> deleted once .bdrv_drain is called.
> 
> Call the qed_need_check_timer_cb manually to update the header
> immediately.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/qed.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/block/qed.c b/block/qed.c
> index 5ea05d4..e9dcb4d 100644
> --- a/block/qed.c
> +++ b/block/qed.c
> @@ -375,6 +375,12 @@ static void bdrv_qed_attach_aio_context(BlockDriverState *bs,
>      }
>  }
>  
> +static void bdrv_qed_drain(BlockDriverState *bs)
> +{
> +    qed_cancel_need_check_timer(bs->opaque);
> +    qed_need_check_timer_cb(bs->opaque);
> +}
> +

Uh oh.

This causes a segfault sometimes, and other times an abort:


   # ./qemu-img create -f qed test.qed 512M
   Formatting 'test.qed', fmt=qed size=536870912 cluster_size=65536

   # ./qemu-io -c "read 0 512M" test.qed
   read 536870912/536870912 bytes at offset 0
   512 MiB, 1 ops; 0.0556 sec (8.988 GiB/sec and 17.9759 ops/sec)
   Segmentation fault (core dumped)


If I run the above qemu-io command with gdb, it will abort in
qed_plug_allocating_write_reqs().

I'd hazard a guess (I have not verified) that it is due to the
qed_header_write() call triggered by the aio flush callback function
qed_clear_need_check().  The aio flush is done inside the
qed_need_check_timer_cb() call.



>  static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
>                           Error **errp)
>  {
> @@ -1676,6 +1682,7 @@ 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)
> -- 
> 2.4.3
> 
>