Re: [Qemu-devel] [PATCH v2 08/11] block: simplify bdrv_drop_intermediate

[Jeff Cody]

On Wed, Jul 17, 2013 at 05:42:13PM +0800, Fam Zheng wrote:
> bdrv_drop_intermediate used a local list to iterate through backing
> chain and delete each BDS. It is simplified while adopting to refcount
> mechanism.
> 

Hi Fam,

The reason for the local list is to keep the BDS deletion
transactional, so it can be rolled back in case of error (see below)

> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  block.c | 71 
> ++++++++++-------------------------------------------------------
>  1 file changed, 11 insertions(+), 60 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 57a3876..499de22 100644
> --- a/block.c
> +++ b/block.c
> @@ -2027,12 +2027,6 @@ BlockDriverState *bdrv_find_overlay(BlockDriverState 
> *active,
>      return overlay;
>  }
>  
> -typedef struct BlkIntermediateStates {
> -    BlockDriverState *bs;
> -    QSIMPLEQ_ENTRY(BlkIntermediateStates) entry;
> -} BlkIntermediateStates;
> -
> -
>  /*
>   * Drops images above 'base' up to and including 'top', and sets the image
>   * above 'top' to have base as its backing file.
> @@ -2062,15 +2056,9 @@ typedef struct BlkIntermediateStates {
>  int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
>                             BlockDriverState *base)
>  {
> -    BlockDriverState *intermediate;
> -    BlockDriverState *base_bs = NULL;
>      BlockDriverState *new_top_bs = NULL;
> -    BlkIntermediateStates *intermediate_state, *next;
>      int ret = -EIO;
>  
> -    QSIMPLEQ_HEAD(states_to_delete, BlkIntermediateStates) states_to_delete;
> -    QSIMPLEQ_INIT(&states_to_delete);
> -
>      if (!top->drv || !base->drv) {
>          goto exit;
>      }
> @@ -2082,58 +2070,21 @@ int bdrv_drop_intermediate(BlockDriverState *active, 
> BlockDriverState *top,
>          goto exit;
>      }
>  
> -    /* special case of new_top_bs->backing_hd already pointing to base - 
> nothing
> -     * to do, no intermediate images */
> -    if (new_top_bs->backing_hd == base) {
> -        ret = 0;
> -        goto exit;
> -    }
> -
> -    intermediate = top;
> -
> -    /* now we will go down through the list, and add each BDS we find
> -     * into our deletion queue, until we hit the 'base'
> -     */
> -    while (intermediate) {
> -        intermediate_state = g_malloc0(sizeof(BlkIntermediateStates));
> -        intermediate_state->bs = intermediate;
> -        QSIMPLEQ_INSERT_TAIL(&states_to_delete, intermediate_state, entry);
> -
> -        if (intermediate->backing_hd == base) {
> -            base_bs = intermediate->backing_hd;
> -            break;
> +    while (new_top_bs->backing_hd && new_top_bs->backing_hd != base) {
> +        BlockDriverState *backing = new_top_bs->backing_hd;
> +        if (backing == NULL) {
> +            goto exit;

If you simplify it until just a while loop that unrefs/deletes the BDS
inside the loop as you navigate the chain, then any error exit leaves
you in a bad state, with a potentially invalid chain.  This is one
such error potential.

>          }
> -        intermediate = intermediate->backing_hd;
> -    }
> -    if (base_bs == NULL) {
> -        /* something went wrong, we did not end at the base. safely
> -         * unravel everything, and exit with error */
> -        goto exit;
> +        new_top_bs->backing_hd = backing->backing_hd;
> +        /* break backing_hd chain before releasing bs, so we don't free all 
> the
> +         * way up the backing chain */
> +        backing->backing_hd = NULL;
> +        bdrv_unref(backing, false);

These two statements, which unlink this BDS from the chain, can't be
undone now, in case of error.

>      }
>  
> -    /* success - we can delete the intermediate states, and link top->base */
> -    ret = bdrv_change_backing_file(new_top_bs, base_bs->filename,
> -                                   base_bs->drv ? base_bs->drv->format_name 
> : "");
> -    if (ret) {
> -        goto exit;
> -    }
> -    if (new_top_bs->backing_hd) {
> -        bdrv_unref(new_top_bs->backing_hd, false);
> -    }
> -    new_top_bs->backing_hd = base_bs;
> -    bdrv_ref(base_bs, false);
> -
> -    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, 
> next) {
> -        /* so that bdrv_close() does not recursively close the chain */
> -        intermediate_state->bs->backing_hd = NULL;
> -        bdrv_delete(intermediate_state->bs);
> -    }

The foreach loop over the list was placed such that there were no more
error paths; we were guaranteed at this point to have been able delete
and unchain each intermediate BDS.

> -    ret = 0;
> -
> +    ret = bdrv_change_backing_file(new_top_bs, base->filename,
> +                                   base->drv ? base->drv->format_name : "");

This is effectively another error path that would cause problems, if
ret < 0.

>  exit:
> -    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, 
> next) {
> -        g_free(intermediate_state);
> -    }
>      return ret;
>  }
>  
> -- 
> 1.8.3.2
> 
>