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

[Fam Zheng <famz@redhat.com>]

On Wed, 07/24 19:16, Jeff Cody wrote:
> 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 <famz@redhat.com>
> > ---
> >  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.
> 

Yes, I'll fix this.

-- 
Fam