Re: [Qemu-devel] [PATCH 11/19] block: Don't notify parents in drain call chain

[Eric Blake <eblake@redhat.com>]

On 12/20/2017 04:34 AM, Kevin Wolf wrote:
> This is in preparation for subtree drains, i.e. drained sections that
> affect not only a single node, but recursively all child nodes, too.
> 
> Calling the parent callbacks for drain is pointless when we just came
> from that parent node recursively and leads to multiple increases of
> bs->quiesce_counter in a single drain call. Don't do it.
> 
> In order for this to work correctly, the parent callback must be called
> for every bdrv_drain_begin/end() call, not only for the outermost one:
> 
> If we have a node N with two parents A and B, recursive draining of A
> should cause the quiesce_counter of B to increased because its child N

either 'to be increased' or 'to increase'

> is drained independently of B. If now B is recursively drained, too, A
> must increase its quiesce_counter because N is drained independently of
> A only now, even if N is going from quiesce_counter 1 to 2.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   include/block/block.h |  4 ++--
>   block.c               | 13 +++++++++----
>   block/io.c            | 47 ++++++++++++++++++++++++++++++++++-------------
>   3 files changed, 45 insertions(+), 19 deletions(-)
> 

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