linux-kernel - Re: [PATCH v5 4/8] vfio: Wait for dma-buf invalidation to complete

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Message-ID: <20260130135618.GC2328995@ziepe.ca>
Date: Fri, 30 Jan 2026 09:56:18 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christian König <christian.koenig@....com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Alex Deucher <alexander.deucher@....com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dmitry Osipenko <dmitry.osipenko@...labora.com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Felix Kuehling <Felix.Kuehling@....com>,
	Alex Williamson <alex@...zbot.org>,
	Ankit Agrawal <ankita@...dia.com>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	amd-gfx@...ts.freedesktop.org, virtualization@...ts.linux.dev,
	intel-xe@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, kvm@...r.kernel.org
Subject: Re: [PATCH v5 4/8] vfio: Wait for dma-buf invalidation to complete

On Fri, Jan 30, 2026 at 02:21:08PM +0100, Christian König wrote:

> That would work for me.
> 
> Question is if you really want to do it this way? See usually
> exporters try to avoid blocking such functions.

Yes, it has to be this way, revoke is a synchronous user space
triggered operation around things like FLR or device close. We can't
defer it into some background operation like pm.

> >>>  		}
> >>>  		fput(priv->dmabuf->file);
> >>
> >> This is also extremely questionable. Why doesn't the dmabuf have
> >> a reference while on the linked list?

If we hold a refcount while on the list then the FD can never be
closed.

There is locking protecting the list so that it is safe and close
continues to work right.

Jason