Re: [Patch V8 1/3] iommu: Add support to change default domain of an iommu group

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Will Deacon <will@kernel.org>
Cc: Ashok Raj <ashok.raj@intel.com>,
	Will Deacon <will.deacon@arm.com>,
	iommu@lists.linux-foundation.org,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [Patch V8 1/3] iommu: Add support to change default domain of an iommu group
Date: Fri, 20 Nov 2020 10:11:58 +0800	[thread overview]
Message-ID: <ee19b2ff-1cb6-7db1-9fc9-9e7fb8df5de6@linux.intel.com> (raw)
In-Reply-To: <20201119085303.GA3599@willie-the-truck>

Hi Will,

On 11/19/20 4:53 PM, Will Deacon wrote:
> On Thu, Nov 19, 2020 at 10:18:05AM +0800, Lu Baolu wrote:
>> The original author of this patch series has left Intel. I am now the
>> backup.
> 
> Ok, thanks for letting me know.
> 
>> On 11/18/20 9:51 PM, Will Deacon wrote:
>>> On Fri, Sep 25, 2020 at 12:06:18PM -0700, Ashok Raj wrote:
>>>> From: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
> 
> [...]
> 
>>>> +free_new_domain:
>>>> +	iommu_domain_free(group->default_domain);
>>>> +	group->default_domain = prev_dom;
>>>> +	group->domain = prev_dom;i
>>>
>>> Hmm. This seems to rely on all users of group->default_domain holding the
>>> group->mutex. Have you confirmed that this is the case? There's a funny
>>> use of iommu_group_get() in the exynos IOMMU driver at least.
>>
>> Emm. This change happens within the area with group->mutex held. Or I
>> am not getting your point?
> 
> Yeah, sorry, I wasn't very clear. This code holds the group->mutex, and it
> relies on _anybody_ else who wants to inspect group->default_domain also
> holding that mutex, otherwise they could observe a transient domain pointer
> which we free on the failure path here.

Clear to me now. Thanks for explanation. :-)

Changing default domain through sysfs requires the users to ubind any
driver from the devices in the group. There's a check code and return
failure if this requirement doesn't meet.

So we only need to consider the device release path. device_lock(dev) is
used in this patch to guarantee that no device release happens at the
same time.

> 
> My question is whether or not there is code that inspects
> group->default_domain without group->mutex held? The exynos case doesn't
> obviously hold it, and I'd like to make sure that there aren't others that
> we need to worry about.

I searched the code. The exynos is the only case that inspects
group->default_domain without holding the mutex during run time. It's in
the device release path, so I think it's safe.

> 
> Does that make more sense?

Yes. Thanks!

> 
> Thanks,
> 
> Will
> 

Best regards,
baolu
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