Re: [Qemu-devel] [PATCH 07/16] block: change drain to look only at one child at a time

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/03/2016 04:41, Fam Zheng wrote:
> > bdrv_requests_pending is checking children to also wait until internal
> > requests (such as metadata writes) have completed.  However, checking
> > children is in general overkill because, apart from this special case,
> > the parent's in_flight count will always be incremented by at least one
> > for every request in the child.
> 
> While the patch looks good, I'm not sure I understand this: aren't children's
> requests generated by the child itself, how is parent's in_flight incremented?

What I mean is that there are two cases of bs generating I/O on
bs->file->bs:

1) writes caused by an operation on bs, e.g. a bdrv_aio_write to bs.  In
this case, the bdrv_aio_write increments bs's in_flight count

2) asynchronous metadata writes or flushes.  Such writes can be started
even if bs's in_flight count is zero, but not after the .bdrv_drain
callback has been invoked.  So the correct sequence is:

    - finish current I/O

    - call bdrv_drain callback

    - recurse on children

This is what is needed for QED's callback to work correctly, and I'll
implement it this way in v2.

Paolo