Re: [PATCH 2/4] qcow2: Do not reopen data_file in invalidate_cache

[Eric Blake]

On Wed, Apr 27, 2022 at 01:40:55PM +0200, Hanna Reitz wrote:
> qcow2_co_invalidate_cache() closes and opens the qcow2 file, by calling
> qcow2_close() and qcow2_do_open().  These two functions must thus be
> usable from both a global-state and an I/O context.
> 
> As they are, they are not safe to call in an I/O context, because they
> use bdrv_unref_child() and bdrv_open_child() to close/open the data_file
> child, respectively, both of which are global-state functions.  When
> used from qcow2_co_invalidate_cache(), we do not need to close/open the
> data_file child, though (we do not do this for bs->file or bs->backing
> either), and so we should skip it in the qcow2_co_invalidate_cache()
> path.
> 
> To do so, add a parameter to qcow2_do_open() and qcow2_close() to make
> them skip handling s->data_file, and have qcow2_co_invalidate_cache()
> exempt it from the memset() on the BDRVQcow2State.
> 
> (Note that the QED driver similarly closes/opens the QED image by
> invoking bdrv_qed_close()+bdrv_qed_do_open(), but both functions seem
> safe to use in an I/O context.)
> 
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/945
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  block/qcow2.c | 104 ++++++++++++++++++++++++++++++--------------------
>  1 file changed, 62 insertions(+), 42 deletions(-)

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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