Re: [Qemu-devel] [PATCH v2 3/7] block: implement reference count for BlockDriverState

[Fam Zheng <famz@redhat.com>]

On Tue, 07/30 16:58, Stefan Hajnoczi wrote:
> On Tue, Jul 30, 2013 at 03:52:53PM +0800, Fam Zheng wrote:
> > @@ -1518,6 +1519,9 @@ static void bdrv_move_feature_fields(BlockDriverState *bs_dest,
> >      /* dirty bitmap */
> >      bs_dest->dirty_bitmap       = bs_src->dirty_bitmap;
> >  
> > +    /* reference count */
> > +    bs_dest->refcnt             = bs_src->refcnt;
> > +
> >      /* job */
> >      bs_dest->in_use             = bs_src->in_use;
> >      bs_dest->job                = bs_src->job;
> 
> Not sure this is correct, but then bdrv_swap() is hard to reason
> about... :)
> 
> Imagine an emulated storage controller holds a reference to the
> BlockDriverState.  When we create an external snapshot we'll
> bdrv_swap(old_top, new_top).
> 
> We must not move new_top's refcount into old_top since the old_top
> object is still being referenced by the emulated storage controller.
> When the emulated storage controller does bdrv_unref() we'll hit the
> recount < 0 assertion and be accessing freed memory.
> 
When we swap old_top and new_top, we want to swap all fields except for
these, so we use bdrv_move_feature_fields() to move them back
(bdrv_swap):

    tmp = *bs_new;
    *bs_new = *bs_old;
    *bs_old = tmp;

    /* there are some fields that should not be swapped, move them back */
    bdrv_move_feature_fields(&tmp, bs_old);
    bdrv_move_feature_fields(bs_old, bs_new);
    bdrv_move_feature_fields(bs_new, &tmp);

And I agree that refcnt is one of the fields that shouldn't be moved, so
it's in bdrv_move_feature_fields(). So isn't above right? Without these
lines, it *is* swapped.

> > @@ -4392,6 +4396,24 @@ int64_t bdrv_get_dirty_count(BlockDriverState *bs)
> >      }
> >  }
> >  
> > +/* Get a reference to bs */
> > +void bdrv_ref(BlockDriverState *bs)
> > +{
> > +    bs->refcnt++;
> > +}
> > +
> > +/* Release a previously grabbed reference to bs.
> > + * If after releasing, reference count is zero, the BlockDriverState is
> > + * deleted. */
> > +void bdrv_unref(BlockDriverState *bs)
> > +{
> > +    assert(bs->refcnt > 0);
> > +    if (--bs->refcnt == 0) {
> > +        bdrv_close(bs);
> > +        bdrv_delete(bs);
> 
> bdrv_delete() already calls bdrv_close() internally, no need to call
> bdrv_close() here.

OK!

-- 
Fam