Re: [Qemu-devel] [PATCH v2 04/13] block/commit: refactor commit to use job callbacks

[Max Reitz <mreitz@redhat.com>]

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On 2018-08-24 00:22, John Snow wrote:
> Use the component callbacks; prepare, abort, and clean.
> 
> NB: prepare is only called when the job has not yet failed;
> and abort can be called after prepare.
> 
> complete -> prepare -> abort -> clean
> complete -> abort -> clean
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  block/commit.c | 90 ++++++++++++++++++++++++++++++++--------------------------
>  1 file changed, 49 insertions(+), 41 deletions(-)
> 
> diff --git a/block/commit.c b/block/commit.c
> index b6e8969877..9bf50385cf 100644
> --- a/block/commit.c
> +++ b/block/commit.c

[...]

> @@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend *bs, BlockBackend *base,

[...]

> +static void commit_abort(Job *job)
> +{
> +    CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
> +    BlockDriverState *top_bs = blk_bs(s->top);
> +
> +    /* Make sure commit_top_bs and top stay around until bdrv_replace_node() */
> +    bdrv_ref(top_bs);
> +    bdrv_ref(s->commit_top_bs);
> +
> +    if (s->base) {
> +        blk_unref(s->base);
>      }
>  
> +    /* free the blockers on the intermediate nodes so that bdrv_replace_nodes
> +     * can succeed */
> +    block_job_remove_all_bdrv(&s->common);
> +
> +    /* If bdrv_drop_intermediate() didn't already do that, remove the commit
> +     * filter driver from the backing chain. Do this as the final step so that
> +     * the 'consistent read' permission can be granted.

I'd suggest rewording this to "If bdrv_drop_intermediate() failed (or
was not invoked at all), remove the commit filter driver from the
backing chain now."

Right now it sounds like maybe bdrv_drop_intermediate() removes the
filter, and maybe it doesn't.  But actually it is supposed to always
remove it -- but it may fail, and then we have to force-remove the filter.

Apart from that:

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

> +     *
> +     * XXX Can (or should) we somehow keep 'consistent read' blocked even
> +     * after the failed/cancelled commit job is gone? If we already wrote
> +     * something to base, the intermediate images aren't valid any more. */
> +    bdrv_child_try_set_perm(s->commit_top_bs->backing, 0, BLK_PERM_ALL,
> +                            &error_abort);
> +    bdrv_replace_node(s->commit_top_bs, backing_bs(s->commit_top_bs),
> +                      &error_abort);
> +
> +    bdrv_unref(s->commit_top_bs);
> +    bdrv_unref(top_bs);
> +}



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