So far the assumption has always been that if we try to inactivate a
node, it is already idle. This doesn't hold true any more if we allow
inactivating exported nodes because we can't know when new external
requests come in.

Drain the node around setting BDRV_O_INACTIVE so that requests can't
start operating on an active node and then in the middle it suddenly
becomes inactive. With this change, it's enough for exports to check
for new requests that they operate on an active node (or, like reads,
are allowed even on an inactive node).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block.c b/block.c
index 7eeb8d076e..1601b25f66 100644
--- a/block.c
+++ b/block.c
@@ -7032,7 +7032,9 @@ bdrv_inactivate_recurse(BlockDriverState *bs, bool top_level)
         return -EPERM;
     }
 
+    bdrv_drained_begin(bs);
     bs->open_flags |= BDRV_O_INACTIVE;
+    bdrv_drained_end(bs);
 
     /*
      * Update permissions, they may differ for inactive nodes.
-- 
2.48.1

On Tue, Feb 04, 2025 at 10:14:02PM +0100, Kevin Wolf wrote:
> So far the assumption has always been that if we try to inactivate a
> node, it is already idle. This doesn't hold true any more if we allow
> inactivating exported nodes because we can't know when new external

'deactivating' sounds nicer than 'inactivating', but I can also see
why you went with 'inactivating' since the flag is centered around the
notion of INACTIVE.

> requests come in.
> 
> Drain the node around setting BDRV_O_INACTIVE so that requests can't
> start operating on an active node and then in the middle it suddenly
> becomes inactive. With this change, it's enough for exports to check
> for new requests that they operate on an active node (or, like reads,
> are allowed even on an inactive node).
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c | 2 ++
>  1 file changed, 2 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/block.c b/block.c
> index 7eeb8d076e..1601b25f66 100644
> --- a/block.c
> +++ b/block.c
> @@ -7032,7 +7032,9 @@ bdrv_inactivate_recurse(BlockDriverState *bs, bool top_level)
>          return -EPERM;
>      }
>  
> +    bdrv_drained_begin(bs);
>      bs->open_flags |= BDRV_O_INACTIVE;
> +    bdrv_drained_end(bs);
>  
>      /*
>       * Update permissions, they may differ for inactive nodes.
> -- 
> 2.48.1
> 
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org