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

[Kevin Wolf <kwolf@redhat.com>]

Am 27.04.2022 um 13:40 hat Hanna Reitz geschrieben:
> 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>

This feels a bit like a hack, and we'll have to change it again if we
ever want to allow changing the data_file with reopen. But it should do
the job for now.

Kevin