Re: [Qemu-devel] [PATCH 3/8] block: implement reference count for BlockDriverState

[Fam Zheng <famz@redhat.com>]

On Thu, 07/25 09:15, Jeff Cody wrote:
> On Thu, Jul 25, 2013 at 05:01:41PM +0800, Fam Zheng wrote:
> > Introduce bdrv_ref/bdrv_unref to manage the lifecycle of
> > BlockDriverState. They are unused for now but will used to replace
> > bdrv_delete() later.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block.c                   | 22 ++++++++++++++++++++++
> >  include/block/block.h     |  2 ++
> >  include/block/block_int.h |  1 +
> >  3 files changed, 25 insertions(+)
> > 
> > diff --git a/block.c b/block.c
> > index 6cd39fa..6f7ad7f 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -306,6 +306,7 @@ BlockDriverState *bdrv_new(const char *device_name)
> >      bdrv_iostatus_disable(bs);
> >      notifier_list_init(&bs->close_notifiers);
> >      notifier_with_return_list_init(&bs->before_write_notifiers);
> > +    bs->refcnt = 1;
> >  
> >      return bs;
> >  }
> > @@ -1511,6 +1512,9 @@ 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;
> > @@ -4385,6 +4389,24 @@ 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);
> > +    }
> > +}
> 
> The problem with this is that a caller to bdrv_unref() has no
> way of knowing after calling bdrv_unref() if bs is still valid.  We
> can't just set bs to NULL after calling bdrv_unref() as with
> bdrv_delete(), because now it may not have been freed.
> 
By calling bdrv_unref, it means the caller is not going to use bs any
more.  In other words, bdrv_unref() is a bdrv_delete() as seen by the
caller, if bs is still valid pointer after unref, it's no longer safe
for the caller: it can be freed by other code, in any time, but the
caller can't know.

> Maybe bdrv_unref should either return the current bs pointer, or
> alternatively accept as its argument a pointer to the BDS pointer:
> 
> void bdrv_unref(BlockDriverState **bs)
> {
>     assert(*bs->refcnt > 0);
>     if (--*bs->refcnt == 0) {
>         bdrv_close(*bs);
>         bdrv_delete(*bs);
>         *bs = NULL;
>     }
> }
> 
> Of course, all callers would need to then check for NULL.
> 
> Also, do we need to call bdrv_close() in here?  In bdrv_delete(),
> bdrv_close() is called prior to the free.
> 
Yes, it can be omited.

> > +
> >  void bdrv_set_in_use(BlockDriverState *bs, int in_use)
> >  {
> >      assert(bs->in_use != in_use);
> > diff --git a/include/block/block.h b/include/block/block.h
> > index 742fce5..b33ef62 100644
> > --- a/include/block/block.h
> > +++ b/include/block/block.h
> > @@ -356,6 +356,8 @@ int64_t bdrv_get_dirty_count(BlockDriverState *bs);
> >  void bdrv_enable_copy_on_read(BlockDriverState *bs);
> >  void bdrv_disable_copy_on_read(BlockDriverState *bs);
> >  
> > +void bdrv_ref(BlockDriverState *bs);
> > +void bdrv_unref(BlockDriverState *bs);
> >  void bdrv_set_in_use(BlockDriverState *bs, int in_use);
> >  int bdrv_in_use(BlockDriverState *bs);
> >  
> > diff --git a/include/block/block_int.h b/include/block/block_int.h
> > index c6ac871..a282d56 100644
> > --- a/include/block/block_int.h
> > +++ b/include/block/block_int.h
> > @@ -294,6 +294,7 @@ struct BlockDriverState {
> >      BlockDeviceIoStatus iostatus;
> >      char device_name[32];
> >      HBitmap *dirty_bitmap;
> > +    int refcnt;
> >      int in_use; /* users other than guest access, eg. block migration */
> >      QTAILQ_ENTRY(BlockDriverState) list;
> >  
> > -- 
> > 1.8.3.2
> > 
> > 

-- 
Fam