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From: Will Deacon <will@kernel.org>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>,
	iommu@lists.linux-foundation.org,
	Robin Murphy <robin.murphy@arm.com>,
	Ashok Raj <ashok.raj@intel.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 11:03:55 +0000	[thread overview]
Message-ID: <20201120110355.GA6151@willie-the-truck> (raw)
In-Reply-To: <ee19b2ff-1cb6-7db1-9fc9-9e7fb8df5de6@linux.intel.com>

On Fri, Nov 20, 2020 at 10:11:58AM +0800, Lu Baolu wrote:
> On 11/19/20 4:53 PM, Will Deacon wrote:
> > On Thu, Nov 19, 2020 at 10:18:05AM +0800, Lu Baolu wrote:
> > > 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.

Aha, thanks. Please can you add a comment for future reference?

> 
> > 
> > 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.

Great, thanks for looking.

Will
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  reply	other threads:[~2020-11-20 11:04 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-25 19:06 [Patch V8 0/3] iommu: Add support to change default domain of an iommu group Ashok Raj
2020-09-25 19:06 ` [Patch V8 1/3] " Ashok Raj
2020-11-18 13:51   ` Will Deacon
2020-11-19  2:18     ` Lu Baolu
2020-11-19  8:53       ` Will Deacon
2020-11-20  2:11         ` Lu Baolu
2020-11-20 11:03           ` Will Deacon [this message]
2020-11-20 11:27   ` Shameerali Kolothum Thodi
2020-11-20 13:09     ` Lu Baolu
2020-09-25 19:06 ` [Patch V8 2/3] iommu: Take lock before reading iommu group default domain type Ashok Raj
2020-09-25 19:06 ` [Patch V8 3/3] iommu: Document usage of "/sys/kernel/iommu_groups/<grp_id>/type" file Ashok Raj
2020-11-18 13:51   ` Will Deacon
2020-11-19  2:32     ` Lu Baolu
2020-11-19  8:55       ` Will Deacon
2020-11-20  2:13         ` Lu Baolu
2020-10-01 12:58 ` [Patch V8 0/3] iommu: Add support to change default domain of an iommu group Joerg Roedel
2020-10-01 13:51   ` Raj, Ashok
2020-11-18 13:52 ` Will Deacon
2020-11-19  2:36   ` Lu Baolu

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