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Date: Tue, 30 Jul 2013 16:58:48 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20130730145848.GE9018@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 3/7] block: implement reference count
 for BlockDriverState
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To: Fam Zheng <famz@redhat.com>
Cc: kwolf@redhat.com, jcody@redhat.com, qemu-devel@nongnu.org

On Tue, Jul 30, 2013 at 03:52:53PM +0800, Fam Zheng wrote:
> @@ -1518,6 +1519,9 @@ 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;

Not sure this is correct, but then bdrv_swap() is hard to reason
about... :)

Imagine an emulated storage controller holds a reference to the
BlockDriverState.  When we create an external snapshot we'll
bdrv_swap(old_top, new_top).

We must not move new_top's refcount into old_top since the old_top
object is still being referenced by the emulated storage controller.
When the emulated storage controller does bdrv_unref() we'll hit the
recount < 0 assertion and be accessing freed memory.

> @@ -4392,6 +4396,24 @@ 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);

bdrv_delete() already calls bdrv_close() internally, no need to call
bdrv_close() here.