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From: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Mark Rutland <Mark.Rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	Will Deacon <Will.Deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"robh+dt@kernel.org" <robh+dt@kernel.org>,
	Robin Murphy <Robin.Murphy@arm.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator
Date: Wed, 12 Jun 2019 12:30:05 +0100
Message-ID: <f68f4ccb-1422-4f93-dc9c-2bcdf61c9ed4@arm.com> (raw)
In-Reply-To: <20190611101052.35af46df@jacob-builder>

On 11/06/2019 18:10, Jacob Pan wrote:
>> The issue is theoretical at the moment because no users do this, but
>> I'd be more comfortable taking the xa_lock, which prevents a
>> concurrent xa_erase()+free(). (I commented on your v3 but you might
>> have missed it)
>>
> Did you reply to my v3? I did not see it. I only saw your comments about
> v3 in your commit message.

My fault, I sneaked the comments in a random reply three levels down the
thread:
https://lore.kernel.org/linux-iommu/836caf0d-699e-33ba-5303-b1c9c949c9ca@arm.com/

(Great, linux-iommu is indexed by lore! I won't have to Cc lkml anymore)

>>>> +	ioasid_data = xa_load(&ioasid_xa, ioasid);
>>>> +	if (ioasid_data)
>>>> +		rcu_assign_pointer(ioasid_data->private, data);  
>>> it is good to publish and have barrier here. But I just wonder even
>>> for weakly ordered machine, this pointer update is quite far away
>>> from its data update.  
>>
>> I don't know, it could be right before calling ioasid_set_data():
>>
>> 	mydata = kzalloc(sizeof(*mydata));
>> 	mydata->ops = &my_ops;			(1)
>> 	ioasid_set_data(ioasid, mydata);
>> 		... /* no write barrier here */
>> 		data->private = mydata;		(2)
>>
>> And then another thread calls ioasid_find():
>>
>> 	mydata = ioasid_find(ioasid);
>> 	if (mydata)
>> 		mydata->ops->do_something();
>>
>> On a weakly ordered machine, this thread could observe the pointer
>> assignment (2) before the ops assignment (1), and dereference NULL.
>> Using rcu_assign_pointer() should fix that
>>
> I agree it is better to have the barrier. Just thought there is already
> a rcu_read_lock() in xa_load() in between. rcu_read_lock() may have
> barrier in some case but better not count on it. 

Yes, and even if rcu_read_lock() provided a barrier I don't think it
would be sufficient, because acquire semantics don't guarantee that
prior writes appear to happen before the barrier, only the other way
round. A lock operation with release semantics, for example
spin_unlock(), should work.

Thanks,
Jean

> No issues here. I will
> integrate this in the next version.
> 
>> Thanks,
>> Jean
> 
> [Jacob Pan]
> 


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Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-10 18:47 [PATCH 0/8] iommu: Add auxiliary domain and PASID support to Arm SMMUv3 Jean-Philippe Brucker
2019-06-10 18:47 ` [PATCH 1/8] iommu: Add I/O ASID allocator Jean-Philippe Brucker
2019-06-11  9:36   ` Jonathan Cameron
2019-06-11 14:35     ` Jean-Philippe Brucker
2019-06-11 18:13       ` Jacob Pan
2019-06-18 14:22         ` Jean-Philippe Brucker
2019-06-18 17:05           ` Jacob Pan
2019-06-19 14:26             ` Jean-Philippe Brucker
2019-06-11 12:26   ` Jacob Pan
2019-06-11 14:37     ` Jean-Philippe Brucker
2019-06-11 17:10       ` Jacob Pan
2019-06-12 11:30         ` Jean-Philippe Brucker [this message]
2019-06-10 18:47 ` [PATCH 2/8] dt-bindings: document PASID property for IOMMU masters Jean-Philippe Brucker
2019-07-08  7:58   ` Auger Eric
2019-06-10 18:47 ` [PATCH 3/8] iommu/arm-smmu-v3: Support platform SSID Jean-Philippe Brucker
2019-06-11  9:42   ` Jonathan Cameron
2019-06-11 14:35     ` Jean-Philippe Brucker
2019-06-18 18:08   ` Will Deacon
2019-06-19 11:53     ` Jean-Philippe Brucker
2019-07-08  7:58   ` Auger Eric
2019-06-10 18:47 ` [PATCH 4/8] iommu/arm-smmu-v3: Add support for Substream IDs Jean-Philippe Brucker
2019-06-11 10:19   ` Jonathan Cameron
2019-06-11 14:35     ` Jean-Philippe Brucker
2019-06-26 18:00   ` Will Deacon
2019-07-04  9:33     ` Jean-Philippe Brucker
2019-07-08 15:31   ` Auger Eric
2019-06-10 18:47 ` [PATCH 5/8] iommu/arm-smmu-v3: Add second level of context descriptor table Jean-Philippe Brucker
2019-06-11 10:24   ` Jonathan Cameron
2019-07-08 15:13   ` Auger Eric
2019-06-10 18:47 ` [PATCH 6/8] iommu/arm-smmu-v3: Support auxiliary domains Jean-Philippe Brucker
2019-06-26 17:59   ` Will Deacon
2019-07-05 16:29     ` Jean-Philippe Brucker
2019-06-10 18:47 ` [PATCH 7/8] iommu/arm-smmu-v3: Improve add_device() error handling Jean-Philippe Brucker
2019-07-08  7:58   ` Auger Eric
2019-06-10 18:47 ` [PATCH 8/8] iommu/arm-smmu-v3: Add support for PCI PASID Jean-Philippe Brucker
2019-06-11 10:45   ` Jonathan Cameron
2019-06-11 14:35     ` Jean-Philippe Brucker
2019-07-08  7:58   ` Auger Eric

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