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

[Stefan Hajnoczi]

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.