Re: [PATCH v2 03/36] block: bdrv_append(): don't consume reference

From: Kevin Wolf <kwolf@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Cc: qemu-block@nongnu.org, armbru@redhat.com, qemu-devel@nongnu.org,
	mreitz@redhat.com, den@openvz.org, jsnow@redhat.com
Subject: Re: [PATCH v2 03/36] block: bdrv_append(): don't consume reference
Date: Mon, 18 Jan 2021 15:18:20 +0100	[thread overview]
Message-ID: <20210118141820.GD11555@merkur.fritz.box> (raw)
In-Reply-To: <20201127144522.29991-4-vsementsov@virtuozzo.com>

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 <vsementsov@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