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

From: Alberto Garcia
Subject: Re: [Qemu-block] [PATCH v11 01/19] block: Add bdrv_drain_all_{begin, end}()
Date: Thu, 20 Oct 2016 10:25:30 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

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
>> resumed.
>> 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
> transactions.
> 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


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