Re: [PATCH] docs/devel: Prohibit calling object_unparent() for memory region

[Michael S. Tsirkin]

On Thu, Aug 29, 2024 at 02:46:48PM +0900, Akihiko Odaki wrote:
> Previously it was allowed to call object_unparent() for a memory region
> in instance_finalize() of its parent. However, such a call typically
> has no effect because child objects get unparented before
> instance_finalize().
> 
> Worse, memory regions typically gets finalized when they get unparented
> before instance_finalize(). This means calling object_unparent() for
> them in instance_finalize() is to call the function for an object
> already finalized, which should be avoided.
> 
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>


Acked-by: Michael S. Tsirkin <mst@redhat.com>

who's applying this? Paolo?

> ---
>  docs/devel/memory.rst | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/docs/devel/memory.rst b/docs/devel/memory.rst
> index 69c5e3f914ac..83760279e3db 100644
> --- a/docs/devel/memory.rst
> +++ b/docs/devel/memory.rst
> @@ -168,11 +168,10 @@ and VFIOQuirk in hw/vfio/pci.c.
>  
>  You must not destroy a memory region as long as it may be in use by a
>  device or CPU.  In order to do this, as a general rule do not create or
> -destroy memory regions dynamically during a device's lifetime, and only
> -call object_unparent() in the memory region owner's instance_finalize
> -callback.  The dynamically allocated data structure that contains the
> -memory region then should obviously be freed in the instance_finalize
> -callback as well.
> +destroy memory regions dynamically during a device's lifetime, and do not
> +call object_unparent().  The dynamically allocated data structure that 
> contains
> +the memory region then should be freed in the instance_finalize callback, 
> which
> +is called after it gets unparented.
>  
>  If you break this rule, the following situation can happen:
>  
> @@ -199,8 +198,9 @@ but nevertheless it is used in a few places.
>  
>  For regions that "have no owner" (NULL is passed at creation time), the
>  machine object is actually used as the owner.  Since instance_finalize is
> -never called for the machine object, you must never call object_unparent
> -on regions that have no owner, unless they are aliases or containers.
> +never called for the machine object, you must never free regions that have no
> +owner, unless they are aliases or containers, which you can manually call
> +object_unparent() for.
>  
>  
>  Overlapping regions and priority
> 
> ---
> base-commit: 31669121a01a14732f57c49400bc239cf9fd505f
> change-id: 20240829-memory-cfd3ee0af44d
> 
> Best regards,
> -- 
> Akihiko Odaki <akihiko.odaki@daynix.com>