linux-kernel - RE: [PATCH v11 12/16] iommu: Use refcount for fault data access

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Date: Mon, 5 Feb 2024 08:37:20 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>, "Will
 Deacon" <will@...nel.org>, Robin Murphy <robin.murphy@....com>, "Jason
 Gunthorpe" <jgg@...pe.ca>, Jean-Philippe Brucker <jean-philippe@...aro.org>,
	Nicolin Chen <nicolinc@...dia.com>
CC: "Liu, Yi L" <yi.l.liu@...el.com>, Jacob Pan
	<jacob.jun.pan@...ux.intel.com>, Longfang Liu <liulongfang@...wei.com>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>, Joel Granados <j.granados@...sung.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Jason Gunthorpe <jgg@...dia.com>
Subject: RE: [PATCH v11 12/16] iommu: Use refcount for fault data access

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Tuesday, January 30, 2024 4:09 PM
> 
> The per-device fault data structure stores information about faults
> occurring on a device. Its lifetime spans from IOPF enablement to
> disablement. Multiple paths, including IOPF reporting, handling, and
> responding, may access it concurrently.
> 
> Previously, a mutex protected the fault data from use after free. But
> this is not performance friendly due to the critical nature of IOPF
> handling paths.
> 
> Refine this with a refcount-based approach. The fault data pointer is
> obtained within an RCU read region with a refcount. The fault data
> pointer is returned for usage only when the pointer is valid and a
> refcount is successfully obtained. The fault data is freed with
> kfree_rcu(), ensuring data is only freed after all RCU critical regions
> complete.
> 
> An iopf handling work starts once an iopf group is created. The handling
> work continues until iommu_page_response() is called to respond to the
> iopf and the iopf group is freed. During this time, the device fault
> parameter should always be available. Add a pointer to the device fault
> parameter in the iopf_group structure and hold the reference until the
> iopf_group is freed.
> 
> Make iommu_page_response() static as it is only used in io-pgfault.c.
> 
> Co-developed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Tested-by: Yan Zhao <yan.y.zhao@...el.com>

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

>   * struct iommu_fault_param - per-device IOMMU fault data
>   * @lock: protect pending faults list
> + * @users: user counter to manage the lifetime of the data
> + * @ruc: rcu head for kfree_rcu()

s/ruc/rcu