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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: Fri, 21 Oct 2016 21:03:45 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Fri 21 Oct 2016 08:55:51 PM CEST, John Snow <address@hidden> 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
>> BlockDriverState.
> 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?

Yeah, block-commit for example (see commit_start()), and block-stream
after this series.


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