linux-kernel - Re: [PATCH v6 01/11] iommu: Add dma ownership management interfaces

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Date:   Mon, 21 Feb 2022 12:02:00 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     baolu.lu@...ux.intel.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, kvm@...r.kernel.org,
        rafael@...nel.org, David Airlie <airlied@...ux.ie>,
        linux-pci@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Will Deacon <will@...nel.org>,
        Stuart Yoder <stuyoder@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, Li Yang <leoyang.li@....com>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v6 01/11] iommu: Add dma ownership management interfaces

On 2/19/22 3:31 PM, Christoph Hellwig wrote:
> The overall API and patch looks fine, but:
> 
>> + * iommu_group_dma_owner_claimed() - Query group dma ownership status
>> + * @group: The group.
>> + *
>> + * This provides status query on a given group. It is racey and only for
>> + * non-binding status reporting.
> 
> s/racey/racy/

Yes.

> 
>> + */
>> +bool iommu_group_dma_owner_claimed(struct iommu_group *group)
>> +{
>> +	unsigned int user;
>> +
>> +	mutex_lock(&group->mutex);
>> +	user = group->owner_cnt;
>> +	mutex_unlock(&group->mutex);
>> +
>> +	return user;
>> +}
>> +EXPORT_SYMBOL_GPL(iommu_group_dma_owner_claimed);
> 
> Still no no need for the lock here.

We've discussed this before. I tend to think that is right.

We don't lose anything with this lock held and it also follows the rule
that all accesses to the internal group structure must be done with the
group->mutex held.

Best regards,
baolu