Re: [Qemu-devel] [PATCH] mirror: Drop permissions on s->target on completion

[Max Reitz <mreitz@redhat.com>]

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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: qemu-stable@nongnu.org
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/mirror.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>


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