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Re: [Qemu-block] [PATCH v11 01/19] block: Add bdrv_drain_all_{begin, end

From: John Snow
Subject: Re: [Qemu-block] [PATCH v11 01/19] block: Add bdrv_drain_all_{begin, end}()
Date: Fri, 21 Oct 2016 14:55:51 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

On 10/20/2016 04:25 AM, Alberto Garcia wrote:
On Wed 19 Oct 2016 07:11:20 PM CEST, Kevin Wolf wrote:

bdrv_drain_all() doesn't allow the caller to do anything after all
pending requests have been completed but before block jobs are

This patch splits bdrv_drain_all() into _begin() and _end() for that
purpose. It also adds aio_{disable,enable}_external() calls to
disable external clients in the meantime.

Signed-off-by: Alberto Garcia <address@hidden>

This looks okay as a first step, possibly enough for this series
(we'll have to review this carefully), but it leaves us with a rather
limited version of bdrv_drain_all_begin/end that excludes many useful
cases. One of them is that John wants to use it around QMP

Specifically, you can't add a new BDS or a new block job in a
drain_all section because then bdrv_drain_all_end() would try to
unpause the new thing even though it has never been paused. Depending
on what else we did with it, this will either corrupt the pause
counters or just directly fail an assertion.

The problem is: do you want to be able to create a new block job and let
it run? Because then you can end up having the same problem that this
patch is trying to prevent if the new job attempts to reopen a


The plan was to create jobs in a pre-started mode and only to engage them after the drained section. Do any jobs re-open a BDS prior to the creation of their coroutine?


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