linux-kernel - Re: [RFC V2] iommu: correct group reference count

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Date:	Tue, 10 Nov 2015 09:24:09 +0800
From:	Peng Fan <van.freenix@...il.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	joro@...tes.org, will.deacon@....com,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V2] iommu: correct group reference count

Hi Alex,
On Mon, Nov 09, 2015 at 08:28:09AM -0700, Alex Williamson wrote:
>On Mon, 2015-11-09 at 14:13 +0800, Peng Fan wrote:
>> The basic flow for iommu_group_for_dev is:
>> iommu_group_get_for_dev
>>         |->  iommu_group_get   : increase reference count by 1.
>>              return group;
>>         |->  ops->device_group : Init/increase reference count to/by 1.
>>              iommu_group_add_device  : Increase reference count by 1.
>>              return group;
>> 
>> We can see that ops->device_group and iommu_group_add_device will together
>> increase the iommu group reference count by 2. Actually we only need 1,
>> but not 2. So we need add iommu_group_put after iommu_group_add_device
>> to make sure iommu_group_get_for_dev only increase reference count by 1.
>

Thanks for comments.
>The premise seems incorrect to me.  In the first case where
>iommu_group_get() provides a group, the reference is increased by 1, but
>the device is already a member of the group and therefore already
>increases the reference count of the group by 1.  The minimum group
>reference at that point is therefore 2.
Yeah. The minimum group reference at that point is 2.

>In the second case, the group
>is created, which gives us our first reference, and the device is added,
>giving us our second reference.  Therefore the minimum reference count
>is also 2.

Yeah.

>If we decrement it, allowing the caller to get the group
>with a single reference and they release that reference, the group will
>be destroyed even though it has a device member.

Thanks. Then Caller of iommu_group_get_for_dev should decrease the count by 1.

Thanks,
Peng.

>One reference to the
>group is for the caller, the other reference is for the device contained
>in the group.  Thanks,
>
>Alex
>
>> Signed-off-by: Peng Fan <van.freenix@...il.com>
>> Cc: Joerg Roedel <joro@...tes.org>
>> Cc: Will Deacon <will.deacon@....com>
>> ---
>> 
>> V1 thread: https://lkml.org/lkml/2015/11/3/304
>> Changes V2:
>>  I did not see the update about device_group when I worked out V1. So
>>  redo the patch and refine commit msg and rebased to latest linus'
>>  linux master tree.
>> 
>>  drivers/iommu/iommu.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index abae363..9c1971b 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -852,10 +852,10 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
>>  	}
>>  
>>  	ret = iommu_group_add_device(group, dev);
>> -	if (ret) {
>> -		iommu_group_put(group);
>> +	iommu_group_put(group);
>> +
>> +	if (ret)
>>  		return ERR_PTR(ret);
>> -	}
>>  
>>  	return group;
>>  }
>
>
>
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