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From: Nicolin Chen <nicoleotsuka@gmail.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	jonathanh@nvidia.com, linux-tegra@vger.kernel.org,
	Dmitry Osipenko <digetx@gmail.com>
Subject: Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device()
Date: Mon, 5 Oct 2020 18:05:46 -0700	[thread overview]
Message-ID: <20201006010546.GB28640@Asurada-Nvidia> (raw)
In-Reply-To: <20201005095754.GJ425362@ulmo>

On Mon, Oct 05, 2020 at 11:57:54AM +0200, Thierry Reding wrote:
> On Fri, Oct 02, 2020 at 11:58:29AM -0700, Nicolin Chen wrote:
> > On Fri, Oct 02, 2020 at 06:02:18PM +0300, Dmitry Osipenko wrote:
> > > 02.10.2020 09:08, Nicolin Chen пишет:
> > > >  static int tegra_smmu_of_xlate(struct device *dev,
> > > >  			       struct of_phandle_args *args)
> > > >  {
> > > > +	struct platform_device *iommu_pdev = of_find_device_by_node(args->np);
> > > > +	struct tegra_mc *mc = platform_get_drvdata(iommu_pdev);
> > > >  	u32 id = args->args[0];
> > > >  
> > > > +	of_node_put(args->np);
> > > 
> > > of_find_device_by_node() takes device reference and not the np
> > > reference. This is a bug, please remove of_node_put().
> > 
> > Looks like so. Replacing it with put_device(&iommu_pdev->dev);
> 
> Putting the put_device() here is wrong, though. You need to make sure
> you keep a reference to it as long as you keep accessing the data that
> is owned by it.

I am confused. You said in the other reply (to Dmitry) that we do
need to put_device(mc->dev), where mc->dev should be the same as
iommu_pdev->dev. But here your comments sounds that we should not
put_device at all since ->probe_device/group_device/attach_dev()
will use it later.

> Like I said earlier, this is a bit weird in this case because we're
> self-referencing, so iommu_pdev->dev is going to stay around as long as
> the SMMU is. However, it might be worth to properly track the lifetime
> anyway just so that the code can serve as a good example of how to do
> things.

What's this "track-the-lifetime"?

> If you decide to go for the shortcut and not track this reference
> properly, then at least you need to add a comment as to why it is safe
> to do in this case. This ensures that readers are away of the
> circumstances and don't copy this bad code into a context where the
> circumstances are different.

I don't quite get this "shortcut" here either...mind elaborating?
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WARNING: multiple messages have this Message-ID (diff)
From: Nicolin Chen <nicoleotsuka@gmail.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: Dmitry Osipenko <digetx@gmail.com>,
	joro@8bytes.org, vdumpa@nvidia.com, jonathanh@nvidia.com,
	linux-tegra@vger.kernel.org, iommu@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device()
Date: Mon, 5 Oct 2020 18:05:46 -0700	[thread overview]
Message-ID: <20201006010546.GB28640@Asurada-Nvidia> (raw)
In-Reply-To: <20201005095754.GJ425362@ulmo>

On Mon, Oct 05, 2020 at 11:57:54AM +0200, Thierry Reding wrote:
> On Fri, Oct 02, 2020 at 11:58:29AM -0700, Nicolin Chen wrote:
> > On Fri, Oct 02, 2020 at 06:02:18PM +0300, Dmitry Osipenko wrote:
> > > 02.10.2020 09:08, Nicolin Chen пишет:
> > > >  static int tegra_smmu_of_xlate(struct device *dev,
> > > >  			       struct of_phandle_args *args)
> > > >  {
> > > > +	struct platform_device *iommu_pdev = of_find_device_by_node(args->np);
> > > > +	struct tegra_mc *mc = platform_get_drvdata(iommu_pdev);
> > > >  	u32 id = args->args[0];
> > > >  
> > > > +	of_node_put(args->np);
> > > 
> > > of_find_device_by_node() takes device reference and not the np
> > > reference. This is a bug, please remove of_node_put().
> > 
> > Looks like so. Replacing it with put_device(&iommu_pdev->dev);
> 
> Putting the put_device() here is wrong, though. You need to make sure
> you keep a reference to it as long as you keep accessing the data that
> is owned by it.

I am confused. You said in the other reply (to Dmitry) that we do
need to put_device(mc->dev), where mc->dev should be the same as
iommu_pdev->dev. But here your comments sounds that we should not
put_device at all since ->probe_device/group_device/attach_dev()
will use it later.

> Like I said earlier, this is a bit weird in this case because we're
> self-referencing, so iommu_pdev->dev is going to stay around as long as
> the SMMU is. However, it might be worth to properly track the lifetime
> anyway just so that the code can serve as a good example of how to do
> things.

What's this "track-the-lifetime"?

> If you decide to go for the shortcut and not track this reference
> properly, then at least you need to add a comment as to why it is safe
> to do in this case. This ensures that readers are away of the
> circumstances and don't copy this bad code into a context where the
> circumstances are different.

I don't quite get this "shortcut" here either...mind elaborating?

  reply	other threads:[~2020-10-06  1:12 UTC|newest]

Thread overview: 92+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-02  6:08 [PATCH v4 0/3] iommu/tegra-smmu: Add PCI support Nicolin Chen
2020-10-02  6:08 ` Nicolin Chen
2020-10-02  6:08 ` [PATCH v4 1/3] iommu/tegra-smmu: Use fwspec in tegra_smmu_(de)attach_dev Nicolin Chen
2020-10-02  6:08   ` Nicolin Chen
2020-10-02 14:22   ` Dmitry Osipenko
2020-10-02 14:22     ` Dmitry Osipenko
2020-10-02 14:52     ` Dmitry Osipenko
2020-10-02 14:52       ` Dmitry Osipenko
2020-10-02 19:56       ` Nicolin Chen
2020-10-02 19:56         ` Nicolin Chen
2020-10-02 14:26   ` Dmitry Osipenko
2020-10-02 14:26     ` Dmitry Osipenko
2020-10-02 14:41   ` Dmitry Osipenko
2020-10-02 14:41     ` Dmitry Osipenko
2020-10-02 19:45     ` Nicolin Chen
2020-10-02 19:45       ` Nicolin Chen
2020-10-02 20:12       ` Dmitry Osipenko
2020-10-02 20:12         ` Dmitry Osipenko
2020-10-02 23:53         ` Nicolin Chen
2020-10-02 23:53           ` Nicolin Chen
2020-10-03  4:01           ` Dmitry Osipenko
2020-10-03  4:01             ` Dmitry Osipenko
2020-10-02  6:08 ` [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device() Nicolin Chen
2020-10-02  6:08   ` Nicolin Chen
2020-10-02 14:22   ` Dmitry Osipenko
2020-10-02 14:22     ` Dmitry Osipenko
2020-10-02 14:58     ` Dmitry Osipenko
2020-10-02 14:58       ` Dmitry Osipenko
2020-10-02 19:53       ` Nicolin Chen
2020-10-02 19:53         ` Nicolin Chen
2020-10-05  9:47         ` Thierry Reding
2020-10-05  9:47           ` Thierry Reding
2020-10-02 14:22   ` Dmitry Osipenko
2020-10-02 14:22     ` Dmitry Osipenko
2020-10-02 14:50     ` Dmitry Osipenko
2020-10-02 14:50       ` Dmitry Osipenko
2020-10-05  9:53       ` Thierry Reding
2020-10-05  9:53         ` Thierry Reding
2020-10-05 10:36         ` Dmitry Osipenko
2020-10-05 10:36           ` Dmitry Osipenko
2020-10-05 11:15           ` Thierry Reding
2020-10-05 11:15             ` Thierry Reding
2020-10-05 13:28             ` Dmitry Osipenko
2020-10-05 13:28               ` Dmitry Osipenko
2020-10-05 14:22               ` Thierry Reding
2020-10-05 14:22                 ` Thierry Reding
2020-10-05  9:51     ` Thierry Reding
2020-10-05  9:51       ` Thierry Reding
2020-10-02 14:23   ` Dmitry Osipenko
2020-10-02 14:23     ` Dmitry Osipenko
2020-10-02 18:01     ` Nicolin Chen
2020-10-02 18:01       ` Nicolin Chen
2020-10-02 18:20       ` Dmitry Osipenko
2020-10-02 18:20         ` Dmitry Osipenko
2020-10-02 15:02   ` Dmitry Osipenko
2020-10-02 15:02     ` Dmitry Osipenko
2020-10-02 18:58     ` Nicolin Chen
2020-10-02 18:58       ` Nicolin Chen
2020-10-05  9:57       ` Thierry Reding
2020-10-05  9:57         ` Thierry Reding
2020-10-06  1:05         ` Nicolin Chen [this message]
2020-10-06  1:05           ` Nicolin Chen
2020-10-08  9:53           ` Thierry Reding
2020-10-08  9:53             ` Thierry Reding
2020-10-08 21:12             ` Nicolin Chen
2020-10-08 21:12               ` Nicolin Chen
2020-10-09 12:25               ` Thierry Reding
2020-10-09 12:25                 ` Thierry Reding
2020-10-09 15:54                 ` Nicolin Chen
2020-10-09 15:54                   ` Nicolin Chen
2020-10-02 15:23   ` Dmitry Osipenko
2020-10-02 15:23     ` Dmitry Osipenko
2020-10-02 16:00     ` Dmitry Osipenko
2020-10-02 16:00       ` Dmitry Osipenko
2020-10-02 16:37       ` Dmitry Osipenko
2020-10-02 16:37         ` Dmitry Osipenko
2020-10-02 16:50         ` Dmitry Osipenko
2020-10-02 16:50           ` Dmitry Osipenko
2020-10-02  6:08 ` [PATCH v4 3/3] iommu/tegra-smmu: Add PCI support Nicolin Chen
2020-10-02  6:08   ` Nicolin Chen
2020-10-02 14:35   ` Dmitry Osipenko
2020-10-02 14:35     ` Dmitry Osipenko
2020-10-02 17:45     ` Nicolin Chen
2020-10-02 17:45       ` Nicolin Chen
2020-10-02 18:04       ` Dmitry Osipenko
2020-10-02 18:04         ` Dmitry Osipenko
2020-10-02 18:04   ` Dmitry Osipenko
2020-10-02 18:04     ` Dmitry Osipenko
2020-10-05 10:04   ` Thierry Reding
2020-10-05 10:04     ` Thierry Reding
2020-10-06  0:54     ` Nicolin Chen
2020-10-06  0:54       ` Nicolin Chen

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