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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Mon, 9 Nov 2015 10:24:14 +0100
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Subject: Re: [Qemu-devel] [PATCH v3 8/9] block: Introduce
	BlockDriver.bdrv_drain callback
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To: Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Ronnie Sahlberg <ronniesahlberg@gmail.com>, Peter Lieven <pl@kamp.de>, qemu-block@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>



On 09/11/2015 03:56, Fam Zheng wrote:
> +    if (bs->drv && bs->drv->bdrv_drain) {
> +        bs->drv->bdrv_drain(bs);
> +    }
> +    QLIST_FOREACH(child, &bs->children, next) {
> +        BlockDriverState *cbs = child->bs;
> +        if (cbs->drv && cbs->drv->bdrv_drain) {
> +            cbs->drv->bdrv_drain(bs);
> +        }
> +    }

I think this is not enough, because the children could have children as
well.

Perhaps you can do it like bdrv_flush: one function does the call to
bdrv_drain and the recursion on children; bdrv_drain calls that one
function and then does the "while (busy)" loop.

Paolo