Re: [Qemu-devel] [PATCH v3 4/4] migration: use bdrv_drain_all_begin/end() instead bdrv_drain_all()

[Eric Blake <eblake@redhat.com>]

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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 <kwolf@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  migration/savevm.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

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


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