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Re: [Qemu-devel] [PATCH v3 2/2] virtio-blk: Use blk_drain() to drain IO

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v3 2/2] virtio-blk: Use blk_drain() to drain IO requests
Date: Wed, 01 Jul 2015 17:52:27 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Stefan Hajnoczi <address@hidden> writes:

> On Mon, Jun 29, 2015 at 08:10:20AM +0200, Markus Armbruster wrote:
>> Alexander Yarygin <address@hidden> writes:
>> > Markus Armbruster <address@hidden> writes:
>> > * Ignorant answer: I was told that the bdrv_drain_all()'s comment is
>> >   obsolete and we can use bdrv_drain(). Here is a link to the old
>> >   thread: http://marc.info/?l=qemu-devel&m=143154211017926&w=2.
>> Kevin, Stefan, if the comment has become wrong, it needs to be redone.
>> Who's going to take care of it?
> I couldn't think of a scenario where this patch is unsafe.
> The danger is that the I/O code path depends on something outside the
> AioContext.  In that case you block in aio_poll(aio_context, true)
> forever without making progress.  The thing the I/O request depends on
> will never finish.
> Code that accesses multiple BDSes puts them into the same AioContext, so
> this should not be a problem in practice.

Stefan, could you rework the bdrv_drain()'s to spell out how it can be
used safely?

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