linux-kernel - Re: [PATCH v2 4/7] iommu: Let iommu.strict override ops->def_domain

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Message-ID: <73a033d6-c1fe-c012-4648-774aa5b93a61@arm.com>
Date:   Wed, 7 Dec 2022 14:18:36 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Gerd Bayer <gbayer@...ux.ibm.com>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Wenjia Zhang <wenjia@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, linux-kernel@...r.kernel.org,
        Julian Ruess <julianr@...ux.ibm.com>
Subject: Re: [PATCH v2 4/7] iommu: Let iommu.strict override
 ops->def_domain_type

On 2022-12-07 13:23, Jason Gunthorpe wrote:
> On Wed, Dec 07, 2022 at 09:18:19PM +0800, Baolu Lu wrote:
> 
>>> -	/* Check if the device in the group still has a driver bound to it */
>>> -	device_lock(dev);
>>
>> With device_lock() removed, this probably races with the
>> iommu_release_device() path? group->mutex seems insufficient to avoid
>> the race. Perhaps I missed anything.
> 
> This path only deals with group, so there is no 'dev' and no race with
> removal.

If we can now use the ownership mechanism to enforce the required 
constraints for change_dev_def_domain, that would be worthwhile (and a 
lot clearer) as a separate patch in its own right.

Thanks,
Robin.

> Later on we obtain the group mutex and then extract the first device
> from the group list as a representative device of the group - eg to
> perform iommu_domain allocation.
> 
> Under the group mutex devices on the device list cannot become
> invalid.
> 
> It is the same reasoning we use in other places that iterate over the
> group device list under lock.
> 
> Jason