linux-kernel - Re: [RFC] iommu: arm-smmu: correct reference count

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Message-ID: <20151106122338.GI6087@arm.com>
Date:	Fri, 6 Nov 2015 12:23:39 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peng Fan <b51431@...escale.com>
Cc:	Peng Fan <van.freenix@...il.com>, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC] iommu: arm-smmu: correct reference count

On Wed, Nov 04, 2015 at 10:48:28AM +0800, Peng Fan wrote:
> On Tue, Nov 03, 2015 at 01:17:34PM +0000, Will Deacon wrote:
> >On Tue, Nov 03, 2015 at 08:59:17PM +0800, Peng Fan wrote:
> >> iommu_group_alloc will initialize the reference count for group to 1.
> >> iommu_group_add_device also increase the group reference count,
> >> if nothing bad happends. And we need to add iommu_group_put to
> >> decrease the reference count for group.
> >> 
> >> Signed-off-by: Peng Fan <van.freenix@...il.com>
> >> Cc: Will Deacon <will.deacon@....com>
> >> Cc: Joerg Roedel <joro@...tes.org>
> >> ---
> >> 
> >> Not sure whether my understanding is correct or not. I checked rockchip-iommu.c
> >> exynos-iommu.c and fsl_pamu_domain.c, and they all have iommu_group_put after
> >> iommu_group_add_device.
> >
> >Doesn't this pair up with the iommu_group_remove_device in
> >arm_smmu_remove_device? Are you actually seeing an issue in practice?
> 
> In arm_smmu_add_platform_device,
> iommu_group_alloc --> group->device_kobj ref count will be init to 1.
> iommu_group_add_device --> group->device_kobj ref count will be added 1 to 2.
> 
> In arm_smmu_remove_device:
> iommu_group_remove_device --> Decrease group->device_kobj ref count by 1.
> After arm_smmu_remove_device, the ref count of group->device_kobj is not 0.
> 
> So I think need to add iommu_group_put after iommu_group_add_device.
> If I am wrong, please correct me.

Ok, I see what you're saying. iommu_group_get_for_dev has exactly the
same issue, so could you update arm-smmu-v3.c as well, please?

Thanks,

Will
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