linux-kernel - Re: [PATCH v4 10/12] iommu: Prepare IOMMU domain for IOPF

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Date:   Fri, 29 Apr 2022 14:17:23 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 10/12] iommu: Prepare IOMMU domain for IOPF

Hi Jean,

On 2022/4/28 22:47, Jean-Philippe Brucker wrote:
> Hi Baolu,
> 
> On Thu, Apr 21, 2022 at 01:21:19PM +0800, Lu Baolu wrote:
>> +/*
>> + * Get the attached domain for asynchronous usage, for example the I/O
>> + * page fault handling framework. The caller get a reference counter
>> + * of the domain automatically on a successful return and should put
>> + * it with iommu_domain_put() after usage.
>> + */
>> +struct iommu_domain *
>> +iommu_get_domain_for_dev_pasid_async(struct device *dev, ioasid_t pasid)
>> +{
>> +	struct iommu_domain *domain;
>> +	struct iommu_group *group;
>> +
>> +	if (!pasid_valid(pasid))
>> +		return NULL;
>> +
>> +	group = iommu_group_get(dev);
>> +	if (!group)
>> +		return NULL;
>> +
>> +	mutex_lock(&group->mutex);
> 
> There is a possible deadlock between unbind() and the fault handler:
> 
>   unbind()                            iopf_handle_group()
>    mutex_lock(&group->mutex)
>    iommu_detach_device_pasid()
>     iopf_queue_flush_dev()             iommu_get_domain_for_dev_pasid_async()
>      ... waits for IOPF work            mutex_lock(&group->mutex)
> 

Yes, really.

> I was wrong in my previous review: we do have a guarantee that the SVA
> domain does not go away during IOPF handling, because unbind() waits for
> pending faults with iopf_queue_flush_dev() before freeing the domain (or
> for Arm stall, knows that there are no pending faults). So we can just get
> rid of domain->async_users and the group->mutex in IOPF, I think?

Agreed with you. The Intel code does the same thing in its unbind().

Thus, the sva domain's life cycle has already synchronized with IOPF
handling, there's no need for domain->async.

I will drop it in the next version. Thanks you!

Best regards,
baolu