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

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Message-ID: <BN9PR11MB5276092A811C3FA85B1667C88C34A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 15 Aug 2025 08:27:17 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Nicolin Chen <nicolinc@...dia.com>,
	"robin.murphy@....com" <robin.murphy@....com>, "joro@...tes.org"
	<joro@...tes.org>, "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
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	<helgaas@...nel.org>, "etzhao1900@...il.com" <etzhao1900@...il.com>
Subject: RE: [PATCH v3 1/5] iommu: Lock group->mutex in iommu_deferred_attach

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Friday, August 15, 2025 1:10 PM
> 
> On 8/12/25 06:59, Nicolin Chen wrote:
> >
> > +	/*
> > +	 * 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);
> }
> 

As the comment said it's the fast path so avoid locking. your way
implies that every map call needs to acquire the group mutex.