Am 27.11.2020 um 15:44 hat Vladimir Sementsov-Ogievskiy geschrieben:
> We have too much comments for this feature. It seems better just don't
> do it. Most of real users (tests don't count) have to create additional
> reference.
>
> Drop also comment in external_snapshot_prepare:
> - bdrv_append doesn't "remove" old bs in common sense, it sounds
> strange
> - the fact that bdrv_append can fail is obvious from the context
> - the fact that we must rollback all changes in transaction abort is
> known (it's the direct role of abort)
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
> ---
> block.c | 19 +++----------------
> block/backup-top.c | 1 -
> block/commit.c | 1 +
> block/mirror.c | 3 ---
> blockdev.c | 4 ----
> tests/test-bdrv-drain.c | 2 +-
> tests/test-bdrv-graph-mod.c | 2 ++
> 7 files changed, 7 insertions(+), 25 deletions(-)
>
> diff --git a/block.c b/block.c
> index 0dd28f0902..55efef3c9d 100644
> --- a/block.c
> +++ b/block.c
> @@ -3145,11 +3145,6 @@ static BlockDriverState
> *bdrv_append_temp_snapshot(BlockDriverState *bs,
> goto out;
> }
>
> - /* bdrv_append() consumes a strong reference to bs_snapshot
> - * (i.e. it will call bdrv_unref() on it) even on error, so in
> - * order to be able to return one, we have to increase
> - * bs_snapshot's refcount here */
> - bdrv_ref(bs_snapshot);
> bdrv_append(bs_snapshot, bs, &local_err);
> if (local_err) {
> error_propagate(errp, local_err);
> @@ -4608,10 +4603,8 @@ void bdrv_replace_node(BlockDriverState *from,
> BlockDriverState *to,
> *
> * This function does not create any image files.
> *
> - * bdrv_append() takes ownership of a bs_new reference and unrefs it because
> - * that's what the callers commonly need. bs_new will be referenced by the
> old
> - * parents of bs_top after bdrv_append() returns. If the caller needs to
> keep a
> - * reference of its own, it must call bdrv_ref().
> + * Recent update: bdrv_append does NOT eat bs_new reference for now. Drop
> this
> + * comment several moths later.
A comment like this is unusual. Do you think there is a high risk of
somebody introducing a new bdrv_append() in parallel and that they would
read this comment when rebasing their existing patches?
If we do keep the comment: s/for now/now/ (it has recently changed,
we're not intending to change it later) and s/moths/months/.
> */
> void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
> Error **errp)
> @@ -4621,20 +4614,14 @@ void bdrv_append(BlockDriverState *bs_new,
> BlockDriverState *bs_top,
> bdrv_set_backing_hd(bs_new, bs_top, &local_err);
> if (local_err) {
> error_propagate(errp, local_err);
> - goto out;
> + return;
> }
>
> bdrv_replace_node(bs_top, bs_new, &local_err);
> if (local_err) {
> error_propagate(errp, local_err);
> bdrv_set_backing_hd(bs_new, NULL, &error_abort);
> - goto out;
Can we leave a return here just in case that new code will be added at
the end of the function?
> }
> -
> - /* bs_new is now referenced by its new parents, we don't need the
> - * additional reference any more. */
> -out:
> - bdrv_unref(bs_new);
> }
>
> static void bdrv_delete(BlockDriverState *bs)
> diff --git a/block/backup-top.c b/block/backup-top.c
> index fe6883cc97..650ed6195c 100644
> --- a/block/backup-top.c
> +++ b/block/backup-top.c
> @@ -222,7 +222,6 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState
> *source,
>
> bdrv_drained_begin(source);
>
> - bdrv_ref(top);
> bdrv_append(top, source, &local_err);
> if (local_err) {
> error_prepend(&local_err, "Cannot append backup-top filter: ");
> diff --git a/block/commit.c b/block/commit.c
> index 71db7ba747..61924bcf66 100644
> --- a/block/commit.c
> +++ b/block/commit.c
> @@ -313,6 +313,7 @@ void commit_start(const char *job_id, BlockDriverState
> *bs,
> commit_top_bs->total_sectors = top->total_sectors;
>
> bdrv_append(commit_top_bs, top, &local_err);
> + bdrv_unref(commit_top_bs); /* referenced by new parents or failed */
> if (local_err) {
> commit_top_bs = NULL;
> error_propagate(errp, local_err);
> diff --git a/block/mirror.c b/block/mirror.c
> index 8e1ad6eceb..13f7ecc998 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -1605,9 +1605,6 @@ static BlockJob *mirror_start_job(
> bs_opaque = g_new0(MirrorBDSOpaque, 1);
> mirror_top_bs->opaque = bs_opaque;
>
> - /* bdrv_append takes ownership of the mirror_top_bs reference, need to
> keep
> - * it alive until block_job_create() succeeds even if bs has no parent.
> */
> - bdrv_ref(mirror_top_bs);
> bdrv_drained_begin(bs);
> bdrv_append(mirror_top_bs, bs, &local_err);
> bdrv_drained_end(bs);
> diff --git a/blockdev.c b/blockdev.c
> index b5f11c524b..96c96f8ba6 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1587,10 +1587,6 @@ static void external_snapshot_prepare(BlkActionState
> *common,
> goto out;
> }
>
> - /* This removes our old bs and adds the new bs. This is an operation that
> - * can fail, so we need to do it in .prepare; undoing it for abort is
> - * always possible. */
This comment is still relevant, it's unrelated to the bdrv_ref().
> - bdrv_ref(state->new_bs);
> bdrv_append(state->new_bs, state->old_bs, &local_err);
> if (local_err) {
> error_propagate(errp, local_err);
Kevin
