Re: [Qemu-devel] [PATCH v5 05/26] qcow2: Use unsigned addend for update_refcount()

[Eric Blake <eblake@redhat.com>]

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On 12/15/2014 05:50 AM, Max Reitz wrote:
> update_refcount() and qcow2_update_cluster_refcount() currently take a
> signed addend. At least one caller passes a value directly derived from
> an absolute refcount that should be reached ("l2_refcount - 1" in
> expand_zero_clusters_in_l1()). Therefore, the addend should be unsigned
> because unsigned overflow is well-defined in contrast to signed
> overflow. This will be especially important for 64 bit refcounts.
> 
> Because update_refcount() then no longer knows whether the refcount
> should be increased or decreased (which is important for setting the
> refblock-L2-table cache dependency and for overflow/underflow checks),
> it now requires an additional flag which specified exactly that. The
> same applies to qcow2_update_cluster_refcount().
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-cluster.c  |  2 +-
>  block/qcow2-refcount.c | 65 ++++++++++++++++++++++++++++++++------------------
>  block/qcow2.h          |  3 ++-
>  3 files changed, 45 insertions(+), 25 deletions(-)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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