Re: [Qemu-devel] [PATCH v2 08/16] block: Manage backing file references in bdrv_set_backing_hd()

[Fam Zheng <famz@redhat.com>]

On Thu, 10/01 15:13, Kevin Wolf wrote:
> This simplifies the code somewhat, especially when dropping whole
> backing file subchains.
> 
> The exception is the mirroring code that does adventurous things with
> bdrv_swap() and in order to keep it working, I had to duplicate most of
> bdrv_set_backing_hd() locally. We'll get rid again of this ugliness
> shortly.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c               | 35 ++++++++++++++---------------------
>  block/mirror.c        | 16 ++++++++++++----
>  block/stream.c        | 30 +-----------------------------
>  block/vvfat.c         |  6 +++++-
>  include/block/block.h |  1 +
>  5 files changed, 33 insertions(+), 55 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 20a3c5c..c4dcf81 100644
> --- a/block.c
> +++ b/block.c
> @@ -1094,7 +1094,7 @@ static BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
>      return child;
>  }
>  
> -static void bdrv_detach_child(BdrvChild *child)
> +void bdrv_detach_child(BdrvChild *child)
>  {
>      QLIST_REMOVE(child, next);
>      g_free(child);
> @@ -1112,13 +1112,20 @@ void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child)
>      bdrv_unref(child_bs);
>  }
>  
> +/*
> + * Sets the backing file link of a BDS. A new reference is created; callers
> + * which don't need their own reference any more must call bdrv_unref().
> + */

Should we document the reference to bs->backing->bs as well?

>  void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd)
>  {
> +    if (backing_hd) {
> +        bdrv_ref(backing_hd);
> +    }
>  
>      if (bs->backing) {
>          assert(bs->backing_blocker);
>          bdrv_op_unblock_all(bs->backing->bs, bs->backing_blocker);
> -        bdrv_detach_child(bs->backing);
> +        bdrv_unref_child(bs, bs->backing);
>      } else if (backing_hd) {
>          error_setg(&bs->backing_blocker,
>                     "node is used as backing hd of '%s'",
> @@ -1214,7 +1221,10 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
>          goto free_exit;
>      }
>  
> +    /* Hook up the backing file link; drop our reference, bs owns the
> +     * backing_hd reference now */
>      bdrv_set_backing_hd(bs, backing_hd);
> +    bdrv_unref(backing_hd);
>  
>  free_exit:
>      g_free(backing_filename);
> @@ -1891,11 +1901,7 @@ void bdrv_close(BlockDriverState *bs)
>          bs->drv->bdrv_close(bs);
>          bs->drv = NULL;
>  
> -        if (bs->backing) {
> -            BlockDriverState *backing_hd = bs->backing->bs;
> -            bdrv_set_backing_hd(bs, NULL);
> -            bdrv_unref(backing_hd);
> -        }
> +        bdrv_set_backing_hd(bs, NULL);
>  
>          if (bs->file != NULL) {
>              bdrv_unref_child(bs, bs->file);
> @@ -2428,12 +2434,9 @@ int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
>      BlockDriverState *intermediate;
>      BlockDriverState *base_bs = NULL;
>      BlockDriverState *new_top_bs = NULL;
> -    BlkIntermediateStates *intermediate_state, *next;
> +    BlkIntermediateStates *intermediate_state;
>      int ret = -EIO;
>  
> -    QSIMPLEQ_HEAD(states_to_delete, BlkIntermediateStates) states_to_delete;
> -    QSIMPLEQ_INIT(&states_to_delete);
> -
>      if (!top->drv || !base->drv) {
>          goto exit;
>      }
> @@ -2460,7 +2463,6 @@ int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
>      while (intermediate) {
>          intermediate_state = g_new0(BlkIntermediateStates, 1);
>          intermediate_state->bs = intermediate;
> -        QSIMPLEQ_INSERT_TAIL(&states_to_delete, intermediate_state, entry);
>  
>          if (backing_bs(intermediate) == base) {
>              base_bs = backing_bs(intermediate);
> @@ -2483,17 +2485,8 @@ int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
>      }
>      bdrv_set_backing_hd(new_top_bs, base_bs);
>  
> -    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
> -        /* so that bdrv_close() does not recursively close the chain */
> -        bdrv_set_backing_hd(intermediate_state->bs, NULL);
> -        bdrv_unref(intermediate_state->bs);
> -    }
>      ret = 0;
> -
>  exit:
> -    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
> -        g_free(intermediate_state);
> -    }
>      return ret;
>  }
>  
> diff --git a/block/mirror.c b/block/mirror.c
> index 5bb3e36..911c432 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -370,10 +370,18 @@ static void mirror_exit(BlockJob *job, void *opaque)
>          bdrv_swap(s->target, to_replace);
>          if (s->common.driver->job_type == BLOCK_JOB_TYPE_COMMIT) {
>              /* drop the bs loop chain formed by the swap: break the loop then
> -             * trigger the unref from the top one */
> -            BlockDriverState *p = backing_bs(s->base);
> -            bdrv_set_backing_hd(s->base, NULL);
> -            bdrv_unref(p);
> +             * trigger the unref */
> +            /* FIXME This duplicates bdrv_set_backing_hd(), except for the
> +             * actual detach/unref so that the loop can be broken. When
> +             * bdrv_swap() gets replaced, this will become sane again. */
> +            BlockDriverState *backing = s->base->backing->bs;
> +            assert(s->base->backing_blocker);
> +            bdrv_op_unblock_all(backing, s->base->backing_blocker);
> +            error_free(s->base->backing_blocker);
> +            s->base->backing_blocker = NULL;
> +            bdrv_detach_child(s->base->backing);
> +            s->base->backing = NULL;
> +            bdrv_unref(backing);
>          }
>      }
>      if (s->to_replace) {
> diff --git a/block/stream.c b/block/stream.c
> index ba535c5..3f64fa2 100644
> --- a/block/stream.c
> +++ b/block/stream.c
> @@ -52,34 +52,6 @@ static int coroutine_fn stream_populate(BlockDriverState *bs,
>      return bdrv_co_copy_on_readv(bs, sector_num, nb_sectors, &qiov);
>  }
>  
> -static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
> -                                const char *base_id)
> -{
> -    BlockDriverState *intermediate;
> -    intermediate = backing_bs(top);
> -
> -    /* Must assign before bdrv_delete() to prevent traversing dangling pointer
> -     * while we delete backing image instances.
> -     */
> -    bdrv_set_backing_hd(top, base);
> -
> -    while (intermediate) {
> -        BlockDriverState *unused;
> -
> -        /* reached base */
> -        if (intermediate == base) {
> -            break;
> -        }
> -
> -        unused = intermediate;
> -        intermediate = backing_bs(intermediate);
> -        bdrv_set_backing_hd(unused, NULL);
> -        bdrv_unref(unused);
> -    }
> -
> -    bdrv_refresh_limits(top, NULL);
> -}
> -
>  typedef struct {
>      int ret;
>      bool reached_end;
> @@ -101,7 +73,7 @@ static void stream_complete(BlockJob *job, void *opaque)
>              }
>          }
>          data->ret = bdrv_change_backing_file(job->bs, base_id, base_fmt);
> -        close_unused_images(job->bs, base, base_id);
> +        bdrv_set_backing_hd(job->bs, base);
>      }
>  
>      g_free(s->backing_file_str);
> diff --git a/block/vvfat.c b/block/vvfat.c
> index 7c4b0f5..b41055a 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -2923,6 +2923,7 @@ static BlockDriver vvfat_write_target = {
>  static int enable_write_target(BDRVVVFATState *s, Error **errp)
>  {
>      BlockDriver *bdrv_qcow = NULL;
> +    BlockDriverState *backing;
>      QemuOpts *opts = NULL;
>      int ret;
>      int size = sector2cluster(s, s->sector_count);
> @@ -2971,7 +2972,10 @@ static int enable_write_target(BDRVVVFATState *s, Error **errp)
>      unlink(s->qcow_filename);
>  #endif
>  
> -    bdrv_set_backing_hd(s->bs, bdrv_new());
> +    backing = bdrv_new();
> +    bdrv_set_backing_hd(s->bs, backing);
> +    bdrv_unref(backing);
> +
>      s->bs->backing->bs->drv = &vvfat_write_target;
>      s->bs->backing->bs->opaque = g_new(void *, 1);
>      *(void**)s->bs->backing->bs->opaque = s;
> diff --git a/include/block/block.h b/include/block/block.h
> index c6854a6..5d9092c 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -513,6 +513,7 @@ void bdrv_disable_copy_on_read(BlockDriverState *bs);
>  void bdrv_ref(BlockDriverState *bs);
>  void bdrv_unref(BlockDriverState *bs);
>  void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child);
> +void bdrv_detach_child(BdrvChild *child);
>  
>  bool bdrv_op_is_blocked(BlockDriverState *bs, BlockOpType op, Error **errp);
>  void bdrv_op_block(BlockDriverState *bs, BlockOpType op, Error *reason);
> -- 
> 1.8.3.1
> 
>