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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Akihiko Odaki <akihiko.odaki@daynix.com>
Cc: "Peter Maydell" <peter.maydell@linaro.org>,
	"Eduardo Habkost" <eduardo@habkost.net>,
	"Marcel Apfelbaum" <marcel.apfelbaum@gmail.com>,
	"Philippe Mathieu-Daudé" <philmd@linaro.org>,
	"Yanan Wang" <wangyanan55@huawei.com>,
	"John Snow" <jsnow@redhat.com>,
	"BALATON Zoltan" <balaton@eik.bme.hu>,
	"Jiaxun Yang" <jiaxun.yang@flygoat.com>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Daniel Henrique Barboza" <danielhb413@gmail.com>,
	"David Gibson" <david@gibson.dropbear.id.au>,
	"Harsh Prateek Bora" <harshpb@linux.ibm.com>,
	"Alexey Kardashevskiy" <aik@ozlabs.ru>,
	"Alex Bennée" <alex.bennee@linaro.org>,
	"Fabiano Rosas" <farosas@suse.de>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Thomas Huth" <thuth@redhat.com>,
	"Laurent Vivier" <lvivier@redhat.com>,
	qemu-devel@nongnu.org, qemu-block@nongnu.org,
	qemu-ppc@nongnu.org
Subject: Re: [PATCH] docs/devel: Prohibit calling object_unparent() for memory region
Date: Tue, 10 Sep 2024 13:26:49 -0400	[thread overview]
Message-ID: <20240910132635-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20240829-memory-v1-1-ac07af2f4fa5@daynix.com>

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>



  reply	other threads:[~2024-09-10 17:27 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-08-29  5:46 [PATCH] docs/devel: Prohibit calling object_unparent() for memory region Akihiko Odaki
2024-09-10 17:26 ` Michael S. Tsirkin [this message]
2024-09-10 18:21   ` Peter Maydell
2024-10-08 13:33 ` Peter Maydell
2024-10-12  8:07   ` Akihiko Odaki

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