Re: [Qemu-devel] [Qemu-block] [RFC PATCH 0/8] qemu-img convert with copy offloading

[Fam Zheng <famz@redhat.com>]

On Fri, 04/06 13:41, Paolo Bonzini wrote:
> On 05/04/2018 14:55, Stefan Hajnoczi wrote:
> > bdrv_copy_file_range() will invoke bdrv_co_copy_file_range_src() on
> > src[qcow2].  The qcow2 block driver will invoke
> > bdrv_co_copy_file_range_src() on src[file].  The file-posix driver will
> > invoke bdrv_co_copy_file_range_dst() on dst[raw].  The raw driver will
> > invoke bdrv_co_copy_file_range_dst() on dst[file], which sees that
> > src_bds (src[file]) is also file-posix and then goes ahead with
> > copy_file_range(2).
> > 
> > In the case where src[qcow2] is on file-posix but dst[raw] is on iSCSI,
> > the iSCSI .bdrv_co_copy_file_range_dst() call fails with -ENOTSUP and
> > the block layer can fall back to a traditional copy operation.
> > 
> > With this approach src[qcow2] could take a lock or keep track of a
> > serializing request struct so that other requests cannot interfere with
> > the operation, and it's done in a natural way since we remain in the
> > qcow2 function until the entire operation completes.  There's no need
> > for bookkeeping structs or callbacks.
> 
> Could there be AB-BA deadlock if the guest attempts a concurrent copy
> from A to B and from B to A?

I don't think bs_src need to hold its locks when calling into bs_dst for mapping
write ranges. So it should be safe.

Fam