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

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v2 03/36] block: bdrv_append(): don't consume reference
Date: Mon, 18 Jan 2021 20:21:11 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1

18.01.2021 17:18, Kevin Wolf wrote:
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

Drop also comment in external_snapshot_prepare:
  - bdrv_append doesn't "remove" old bs in common sense, it sounds
  - 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?

Or even later, someone may remember that bdrv_append() eat the reference, and 
then face some strange behavior with a new call of bdrv_append(), finally go to 
check the function code and see the new comment.. I don't insist, we can drop 
the comment

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. */
-    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 
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_append(mirror_top_bs, bs, &local_err);
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 
          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().

I described in commit message, why I dislike this comment :) I can keep it of 
course, it's not critical

-    bdrv_ref(state->new_bs);
      bdrv_append(state->new_bs, state->old_bs, &local_err);
      if (local_err) {
          error_propagate(errp, local_err);


Best regards,

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