Re: [Qemu-devel] [PATCH v2 01/11] block: replace in_use with refcnt_soft and refcnt_hard

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Jul 23, 2013 at 06:32:25PM +0800, Fam Zheng wrote:
> On Tue, 07/23 11:36, Stefan Hajnoczi wrote:
> > On Wed, Jul 17, 2013 at 05:42:06PM +0800, Fam Zheng wrote:
> > > Introduce refcnt_soft (soft reference) and refcnt_hard (hard reference)
> > > to BlockDriverState, since in_use mechanism cannot provide proper
> > > management of lifecycle when a BDS is referenced in multiple places
> > > (e.g. pointed to by another bs's backing_hd while also used as a block
> > > job device, in the use case of image fleecing).
> > > 
> > > The original in_use case is considered a "hard reference" in this patch,
> > > where the bs is busy and should not be used in other tasks that require
> > > a hard reference. (However the interface doesn't force this, caller
> > > still need to call bdrv_in_use() to check by itself.).
> > > 
> > > A soft reference is implemented but not used yet. It will be used in
> > > following patches to manage the lifecycle together with hard reference.
> > > 
> > > If bdrv_ref() is called on a BDS, it must be released by exactly the
> > > same numbers of bdrv_unref() with the same "soft/hard" type, and never
> > > call bdrv_delete() directly. If the BDS is only used locally (unnamed),
> > > bdrv_ref/bdrv_unref can be skipped and just use bdrv_delete().
> > 
> > It is risky to keep bdrv_delete() public.  I suggest replacing
> > bdrv_delete() callers with bdrv_unref() and then making bdrv_delete()
> > static in block.c.
> > 
> > This way it is impossible to make the mistake of calling bdrv_delete()
> > on a BDS which has refcnt > 1.
> > 
> > I don't really understand this patch.  There are now two separate
> > refcounts.  They must both reach 0 for deletion to occur.  I think
> > you plan to treat the "hard" refcount like the in_use counter (there
> > should only be 0 or 1 refs) but you don't enforce it.  It seems cleaner
> > to keep in_use separate: let in_use callers take a refcount and also set
> > in_use.
> 
> OK, I like your ideas, make bdrv_delete private is much cleaner. Will
> fix in next revision.
> 
> I plan to make it like this:
> 
>     /* soft ref */
>     void bdrv_{ref,unref}(bs)
> 
>     /* hard ref */
>     bool bdrv_hard_{ref,unref}(bs)
> 
> usage:
>     bs = bdrv_new()
>     <implicit bdrv_ref(bs) called>
>     ...
>     bdrv_unref(bs)
>     <automatically deleted here>
> 
> or with hard ref:
>     bs = bdrv_new()
>     <implicit bdrv_ref() called>
> 
>     bdrv_hard_ref(bs)
>     ...
>     bdrv_hard_unref(bs)
> 
>     bdrv_unref(bs)
>     <automatically deleted here>
> 
> The second bdrv_hard_ref call to a bs returns false, caller check the
> return value.

Why is hard ref necessary when we already have
bdrv_in_use()/bdrv_set_in_use()?