linux-kernel - Re: [PATCH 1/2] iommu/s390: Fix IOMMU groups

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Message-ID: <20170428144030.GG1332@8bytes.org>
Date:   Fri, 28 Apr 2017 16:40:30 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Gerald Schaefer <gerald.schaefer@...ibm.com>
Cc:     Sebastian Ott <sebott@...ux.vnet.ibm.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 1/2] iommu/s390: Fix IOMMU groups

On Fri, Apr 28, 2017 at 03:20:17PM +0200, Gerald Schaefer wrote:
> On Thu, 27 Apr 2017 23:12:32 +0200
> Joerg Roedel <joro@...tes.org> wrote:

> > This is the way to free an iommu-group. It was missing before probably
> > because it was unclear whether the add_device function allocated a group
> > or not. So there was no way to know if it needs to be put again in the
> > remove_device function.
> 
> Hmm, for the reference count it should not matter whether a new group was
> allocated or an existing group found with iommu_group_get(). Our add_device
> callback always gets one reference either from iommu_group_get or _alloc,
> and then another one from iommu_group_add_device(), after which the first
> reference is put again. So there should always be one reference more after
> a successful add_device.

Right, my statement above is wrong. The current code is fine, it gets a
reference to the group with iommu_group_get/iommu_group_alloc, attaches
the device to the group (which takes a reference to the group of its
own), and in the end it drops its local reference.

When the device->group link is broken up in the remove_device function,
that reference is also dropped. So everything is fine. The additional
iommu_group_put() in my patch is wrong.


Regards,

	Joerg