linux-kernel - Re: [PATCH v7 09/28] iommufd/access: Add internal APIs for HW queue to use

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Message-ID: <20250704130839.GP1209783@nvidia.com>
Date: Fri, 4 Jul 2025 10:08:39 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, will@...nel.org,
	bagasdotme@...il.com, robin.murphy@....com, joro@...tes.org,
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	peterz@...radead.org, yi.l.liu@...el.com, mshavit@...gle.com,
	praan@...gle.com, zhangzekun11@...wei.com, iommu@...ts.linux.dev,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
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Subject: Re: [PATCH v7 09/28] iommufd/access: Add internal APIs for HW queue
 to use

On Thu, Jun 26, 2025 at 12:34:40PM -0700, Nicolin Chen wrote:
> The new HW queue object, as an internal iommufd object, wants to reuse the
> struct iommufd_access to pin some iova range in the iopt.
> 
> However, an access generally takes the refcount of an ictx. So, in such an
> internal case, a deadlock could happen when the release of the ictx has to
> wait for the release of the access first when releasing a hw_queue object,
> which could wait for the release of the ictx that is refcounted:
>     ictx --releases--> hw_queue --releases--> access
>       ^                                         |
>       |_________________releases________________v
> 
> To address this, add a set of lightweight internal APIs to unlink the ictx
> and the access, i.e. no ictx refcounting by the access:
>     ictx --releases--> hw_queue --releases--> access
> 
> Then, there's no point in setting the access->ictx. So simply define !ictx
> as an flag for an internal use and add an inline helper.
> 
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  drivers/iommu/iommufd/iommufd_private.h | 23 ++++++++++
>  drivers/iommu/iommufd/device.c          | 59 +++++++++++++++++++++----
>  2 files changed, 73 insertions(+), 9 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason