Re: [PATCH v2] block/stream: Drain subtree around graph change

[Eric Blake <eblake@redhat.com>]

On Thu, Mar 24, 2022 at 03:09:07PM +0100, Hanna Reitz wrote:
> When the stream block job cuts out the nodes between top and base in
> stream_prepare(), it does not drain the subtree manually; it fetches the
> base node, and tries to insert it as the top node's backing node with
> bdrv_set_backing_hd().  bdrv_set_backing_hd() however will drain, and so
> the actual base node might change (because the base node is actually not
> part of the stream job) before the old base node passed to
> bdrv_set_backing_hd() is installed.
> 
...
> 
> Therefore, we need to keep the whole subtree drained in
> stream_prepare(), so that the graph modification it performs is
> effectively atomic, i.e. that the base node it fetches is still the base
> node when bdrv_set_backing_hd() sets it as the top node's backing node.
> 
> Verify this by asserting in said 030's test case that the base node is
> always the top node's immediate backing child when both jobs are done.
> 
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
> v2:
> - Oops, the base can be NULL.  Would have noticed if I had ran all test
>   cases from 030, and not just test_overlapping_5()...
>   That means that keeping a strong reference to the base node must be
>   conditional, based on whether there even is a base node or not.
>   (I mean, technically we do not even need to keep a strong reference to
>   that node, given that we are in a drained section, but I believe it is
>   better style to do it anyway.)

I agree with the conclusion that we don't need a strong reference once
you fix the bigger problem of the lock-by-drain, but that it is better
style to include it anyway.

> ---
>  block/stream.c         | 15 ++++++++++++++-
>  tests/qemu-iotests/030 |  5 +++++
>  2 files changed, 19 insertions(+), 1 deletion(-)

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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