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

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Message-ID: <20250710171515.GS1599700@nvidia.com>
Date: Thu, 10 Jul 2025 14:15:15 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Xu Yilun <yilun.xu@...ux.intel.com>,
	"will@...nel.org" <will@...nel.org>,
	"aneesh.kumar@...nel.org" <aneesh.kumar@...nel.org>,
	"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

On Thu, Jul 10, 2025 at 07:40:58AM +0000, Tian, Kevin wrote:
> > 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?

Probably so, as a followup change would be fine if we don't need a v5

/*
 * Destroy will sleep and wait for wait_cnt to go to zero. This
 * allows concurrent users of the ID to reliably avoid causing a
 * spurious destroy failure. Incrementing this count should either be
 * short lived or be revoked and blocked during pre_destroy
 */
refcount_t wait_cnt;

?

Jason