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Re: [Qemu-block] [Qemu-devel] [PATCH v3 4/4] migration: use bdrv_drain_a

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v3 4/4] migration: use bdrv_drain_all_begin/end() instead bdrv_drain_all()
Date: Mon, 22 May 2017 12:40:10 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0

On 05/22/2017 08:57 AM, Stefan Hajnoczi wrote:
> blk/bdrv_drain_all() only takes effect for a single instant and then
> resumes block jobs, guest devices, and other external clients like the
> NBD server.  This can be handy when performing a synchronous drain
> before terminating the program, for example.
> Monitor commands usually need to quiesce I/O across an entire code
> region so blk/bdrv_drain_all() is not suitable.  They must use
> bdrv_drain_all_begin/end() to mark the region.  This prevents new I/O
> requests from slipping in or worse - block jobs completing and modifying
> the graph.
> I audited other blk/bdrv_drain_all() callers but did not find anything
> that needs a similar fix.  This patch fixes the savevm/loadvm commands.
> Although I haven't encountered a read world issue this makes the code
> safer.
> Suggested-by: Kevin Wolf <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  migration/savevm.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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