linux-kernel - RE: [PATCH v4 3/7] iommufd: Add a pre

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Message-ID: <BN9PR11MB5276CA77E378CB3173CA76888C48A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Thu, 10 Jul 2025 07:40:58 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>, "jgg@...dia.com" <jgg@...dia.com>,
	"jgg@...pe.ca" <jgg@...pe.ca>, "will@...nel.org" <will@...nel.org>,
	"aneesh.kumar@...nel.org" <aneesh.kumar@...nel.org>
CC: "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"joro@...tes.org" <joro@...tes.org>, "robin.murphy@....com"
	<robin.murphy@....com>, "shuah@...nel.org" <shuah@...nel.org>,
	"nicolinc@...dia.com" <nicolinc@...dia.com>, "aik@....com" <aik@....com>,
	"Williams, Dan J" <dan.j.williams@...el.com>, "baolu.lu@...ux.intel.com"
	<baolu.lu@...ux.intel.com>, "Xu, Yilun" <yilun.xu@...el.com>
Subject: RE: [PATCH v4 3/7] iommufd: Add a pre_destroy() op for objects

> From: Xu Yilun <yilun.xu@...ux.intel.com>
> Sent: Wednesday, July 9, 2025 12:03 PM
> 
> Add a pre_destroy() op which gives objects a chance to clear their
> short term users references before destruction. This op is intended for
> external driver created objects (e.g. idev) which does deterministic
> destruction.
> 
> In order to manage the lifecycle of interrelated objects as well as the
> deterministic destruction (e.g. vdev can't outlive idev, and idev
> destruction can't fail), short term users references are allowed to
> live out of an ioctl execution. An immediate use case is, vdev holds
> idev's short term user reference until vdev destruction completes, idev
> leverages existing wait_shortterm mechanism to ensure it is destroyed
> after vdev.
> 
> This extended usage makes the referenced object unable to just wait for
> its reference gone. It needs to actively trigger the reference removal,
> as well as prevent new references before wait. Should implement these
> work in pre_destroy().
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Xu Yilun <yilun.xu@...ux.intel.com>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>

btw is it clearer to rename 'shortterm_users' as 'wait_cnt', as the
meaning now is beyond shortterm and is really about the need of
waiting?