Re: [Qemu-devel] [PATCH 3/8] block: implement reference count for BlockDriverState

[Jeff Cody <jcody@redhat.com>]

On Thu, Jul 25, 2013 at 05:01:41PM +0800, Fam Zheng wrote:
> Introduce bdrv_ref/bdrv_unref to manage the lifecycle of
> BlockDriverState. They are unused for now but will used to replace
> bdrv_delete() later.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block.c                   | 22 ++++++++++++++++++++++
>  include/block/block.h     |  2 ++
>  include/block/block_int.h |  1 +
>  3 files changed, 25 insertions(+)
> 
> diff --git a/block.c b/block.c
> index 6cd39fa..6f7ad7f 100644
> --- a/block.c
> +++ b/block.c
> @@ -306,6 +306,7 @@ BlockDriverState *bdrv_new(const char *device_name)
>      bdrv_iostatus_disable(bs);
>      notifier_list_init(&bs->close_notifiers);
>      notifier_with_return_list_init(&bs->before_write_notifiers);
> +    bs->refcnt = 1;
>  
>      return bs;
>  }
> @@ -1511,6 +1512,9 @@ static void bdrv_move_feature_fields(BlockDriverState *bs_dest,
>      /* dirty bitmap */
>      bs_dest->dirty_bitmap       = bs_src->dirty_bitmap;
>  
> +    /* reference count */
> +    bs_dest->refcnt             = bs_src->refcnt;
> +
>      /* job */
>      bs_dest->in_use             = bs_src->in_use;
>      bs_dest->job                = bs_src->job;
> @@ -4385,6 +4389,24 @@ int64_t bdrv_get_dirty_count(BlockDriverState *bs)
>      }
>  }
>  
> +/* Get a reference to bs */
> +void bdrv_ref(BlockDriverState *bs)
> +{
> +    bs->refcnt++;
> +}
> +
> +/* Release a previously grabbed reference to bs.
> + * If after releasing, reference count is zero, the BlockDriverState is
> + * deleted. */
> +void bdrv_unref(BlockDriverState *bs)
> +{
> +    assert(bs->refcnt > 0);
> +    if (--bs->refcnt == 0) {
> +        bdrv_close(bs);
> +        bdrv_delete(bs);
> +    }
> +}

The problem with this is that a caller to bdrv_unref() has no
way of knowing after calling bdrv_unref() if bs is still valid.  We
can't just set bs to NULL after calling bdrv_unref() as with
bdrv_delete(), because now it may not have been freed.

Maybe bdrv_unref should either return the current bs pointer, or
alternatively accept as its argument a pointer to the BDS pointer:

void bdrv_unref(BlockDriverState **bs)
{
    assert(*bs->refcnt > 0);
    if (--*bs->refcnt == 0) {
        bdrv_close(*bs);
        bdrv_delete(*bs);
        *bs = NULL;
    }
}

Of course, all callers would need to then check for NULL.

Also, do we need to call bdrv_close() in here?  In bdrv_delete(),
bdrv_close() is called prior to the free.

> +
>  void bdrv_set_in_use(BlockDriverState *bs, int in_use)
>  {
>      assert(bs->in_use != in_use);
> diff --git a/include/block/block.h b/include/block/block.h
> index 742fce5..b33ef62 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -356,6 +356,8 @@ int64_t bdrv_get_dirty_count(BlockDriverState *bs);
>  void bdrv_enable_copy_on_read(BlockDriverState *bs);
>  void bdrv_disable_copy_on_read(BlockDriverState *bs);
>  
> +void bdrv_ref(BlockDriverState *bs);
> +void bdrv_unref(BlockDriverState *bs);
>  void bdrv_set_in_use(BlockDriverState *bs, int in_use);
>  int bdrv_in_use(BlockDriverState *bs);
>  
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index c6ac871..a282d56 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -294,6 +294,7 @@ struct BlockDriverState {
>      BlockDeviceIoStatus iostatus;
>      char device_name[32];
>      HBitmap *dirty_bitmap;
> +    int refcnt;
>      int in_use; /* users other than guest access, eg. block migration */
>      QTAILQ_ENTRY(BlockDriverState) list;
>  
> -- 
> 1.8.3.2
> 
>