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Re: [Qemu-block] [PATCH] mirror: Drop permissions on s->target on comple

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH] mirror: Drop permissions on s->target on completion
Date: Mon, 29 May 2017 15:36:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0

On 2017-05-29 14:18, Kevin Wolf wrote:
> This fixes an assertion failure that was triggered by qemu-iotests 129
> on some CI host, while the same test case didn't seem to fail on other
> hosts.
> Essentially the problem is that the blk_unref(s->target) in
> mirror_exit() doesn't necessarily mean that the BlockBackend goes away
> immediately. It is possible that the job completion was triggered nested
> in mirror_drain(), which looks like this:
>     BlockBackend *target = s->target;
>     blk_ref(target);
>     blk_drain(target);
>     blk_unref(target);
> In this case, the write permissions for s->target are retained until
> after blk_drain(), which makes removing mirror_top_bs fail for the
> active commit case (can't have a writable backing file in the chain
> without the filter driver).
> Explicitly dropping the permissions first means that the additional
> reference doesn't hurt and the job can complete successfully even if
> called from the nested blk_drain().
> Cc: address@hidden
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block/mirror.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Reviewed-by: Max Reitz <address@hidden>

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