linux-kernel - Re: [PATCH v3 1/5] iommu: Lock group->mutex in iommu_deferred

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Message-ID: <eb248b73-7fb6-47cd-9e10-5c54ca2391b2@linux.intel.com>
Date: Fri, 15 Aug 2025 13:09:51 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Nicolin Chen <nicolinc@...dia.com>, robin.murphy@....com,
 joro@...tes.org, bhelgaas@...gle.com, jgg@...dia.com
Cc: will@...nel.org, robin.clark@....qualcomm.com, yong.wu@...iatek.com,
 matthias.bgg@...il.com, angelogioacchino.delregno@...labora.com,
 thierry.reding@...il.com, vdumpa@...dia.com, jonathanh@...dia.com,
 rafael@...nel.org, lenb@...nel.org, kevin.tian@...el.com,
 yi.l.liu@...el.com, linux-arm-kernel@...ts.infradead.org,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, linux-mediatek@...ts.infradead.org,
 linux-tegra@...r.kernel.org, linux-acpi@...r.kernel.org,
 linux-pci@...r.kernel.org, patches@...ts.linux.dev, pjaroszynski@...dia.com,
 vsethi@...dia.com, helgaas@...nel.org, etzhao1900@...il.com
Subject: Re: [PATCH v3 1/5] iommu: Lock group->mutex in iommu_deferred_attach

On 8/12/25 06:59, Nicolin Chen wrote:
> The iommu_deferred_attach() is a runtime asynchronous function called by
> iommu-dma function, which could race against other attach functions if it
> accesses something in the dev->iommu_group.
> 
> So, grab the mutex to guard __iommu_attach_device() like other callers.
> 
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>   drivers/iommu/iommu.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 060ebe330ee16..1e0116bce0762 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -2144,10 +2144,17 @@ EXPORT_SYMBOL_GPL(iommu_attach_device);
>   
>   int iommu_deferred_attach(struct device *dev, struct iommu_domain *domain)
>   {
> -	if (dev->iommu && dev->iommu->attach_deferred)
> -		return __iommu_attach_device(domain, dev);
> +	/*
> +	 * This is called on the dma mapping fast path so avoid locking. This is
> +	 * racy, but we have an expectation that the driver will setup its DMAs

Why not making it like this,

int iommu_deferred_attach(struct device *dev, struct iommu_domain *domain)
{
         /* Caller must be a probed driver on dev */
         if (!dev->iommu_group)
                 return 0;

         guard(mutex)(&dev->iommu_group->mutex);
         if (!dev->iommu->attach_deferred)
                 return 0;

         return __iommu_attach_device(domain, dev);
}

?

> +	 * inside probe while being single threaded to avoid racing.
> +	 */
> +	if (!dev->iommu || !dev->iommu->attach_deferred)
> +		return 0;
>   
> -	return 0;
> +	guard(mutex)(&dev->iommu_group->mutex);
> +
> +	return __iommu_attach_device(domain, dev);
>   }
>   
>   void iommu_detach_device(struct iommu_domain *domain, struct device *dev)

Thanks,
baolu