Re: [Qemu-block] [PATCH 12/16] block: Introduce parents list

[Alberto Garcia]

On Tue 29 Sep 2015 02:21:37 PM CEST, Kevin Wolf wrote:
>> > @@ -1090,6 +1090,7 @@ static BdrvChild *bdrv_attach_child(BlockDriverState 
>> > *parent_bs,
>> >  };
>> >  
>> >  QLIST_INSERT_HEAD(_bs->children, child, next);
>> > +QLIST_INSERT_HEAD(_bs->parents, child, next_parent);
>> >  
>> >  return child;
>> >  }
>> 
>> Ok, I'm probably slow today, but what is this used for? :-? And why
>> is it called 'parents'? The list doesn't contain pointers to the
>> parents of child_bs, but to child_bs itself...
>
> It's the list of BdrvChild objects that point to the given
> BlockDriverState.

Ok I see now. I think the names here are misleading, 'parent' means the
BdrvChild object, not parent_bs.

The name child_bs->parents suggests that you can actually iterate over
the BDSs that have child_bs as a child, but that's not the case, it's a
mechanism that allows children to change the pointers that are pointing
at them.

I'm not sure what a better name would be ("reverse refs"?), but at least
I think this deserves a comment.

Berto

Re: [Qemu-block] [PATCH 12/16] block: Introduce parents list

[Kevin Wolf]

Am 28.09.2015 um 15:09 hat Alberto Garcia geschrieben:
> On Thu 17 Sep 2015 03:48:16 PM CEST, Kevin Wolf wrote:
> 
> > @@ -1090,6 +1090,7 @@ static BdrvChild *bdrv_attach_child(BlockDriverState 
> > *parent_bs,
> >  };
> >  
> >  QLIST_INSERT_HEAD(_bs->children, child, next);
> > +QLIST_INSERT_HEAD(_bs->parents, child, next_parent);
> >  
> >  return child;
> >  }
> 
> Ok, I'm probably slow today, but what is this used for? :-? And why is
> it called 'parents'? The list doesn't contain pointers to the parents of
> child_bs, but to child_bs itself...
> 
> I would expect a BdrvChild *parent, with parent->bs = parent_bs.

It's the list of BdrvChild objects that point to the given
BlockDriverState. This is good enough for updating the pointers. If we
ever need the actual parent BDS, we can add a pointer to BdrvChild.

Kevin

Re: [Qemu-block] [PATCH 12/16] block: Introduce parents list

[Max Reitz]

On 17.09.2015 15:48, Kevin Wolf wrote:
> Signed-off-by: Kevin Wolf 
> ---
>  block.c   | 3 +++
>  include/block/block_int.h | 2 ++
>  2 files changed, 5 insertions(+)

Reviewed-by: Max Reitz 



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[Qemu-block] [PATCH 12/16] block: Introduce parents list

[Kevin Wolf]

Signed-off-by: Kevin Wolf 
---
 block.c   | 3 +++
 include/block/block_int.h | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/block.c b/block.c
index 7930f3c..c196f83 100644
--- a/block.c
+++ b/block.c
@@ -1090,6 +1090,7 @@ static BdrvChild *bdrv_attach_child(BlockDriverState 
*parent_bs,
 };
 
 QLIST_INSERT_HEAD(_bs->children, child, next);
+QLIST_INSERT_HEAD(_bs->parents, child, next_parent);
 
 return child;
 }
@@ -1097,6 +1098,7 @@ static BdrvChild *bdrv_attach_child(BlockDriverState 
*parent_bs,
 void bdrv_detach_child(BdrvChild *child)
 {
 QLIST_REMOVE(child, next);
+QLIST_REMOVE(child, next_parent);
 g_free(child);
 }
 
@@ -2037,6 +2039,7 @@ static void bdrv_move_reference_fields(BlockDriverState 
*bs_dest,
 /* keep the same entry in bdrv_states */
 bs_dest->device_list = bs_src->device_list;
 bs_dest->blk = bs_src->blk;
+bs_dest->parents = bs_src->parents;
 
 memcpy(bs_dest->op_blockers, bs_src->op_blockers,
sizeof(bs_dest->op_blockers));
diff --git a/include/block/block_int.h b/include/block/block_int.h
index cfcae52..52ea7c0 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -339,6 +339,7 @@ struct BdrvChild {
 BlockDriverState *bs;
 const BdrvChildRole *role;
 QLIST_ENTRY(BdrvChild) next;
+QLIST_ENTRY(BdrvChild) next_parent;
 };
 
 /*
@@ -445,6 +446,7 @@ struct BlockDriverState {
  * parent node of this node. */
 BlockDriverState *inherits_from;
 QLIST_HEAD(, BdrvChild) children;
+QLIST_HEAD(, BdrvChild) parents;
 
 QDict *options;
 BlockdevDetectZeroesOptions detect_zeroes;
-- 
1.8.3.1