linux-kernel - Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe

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Message-ID: <20201005095754.GJ425362@ulmo>
Date:   Mon, 5 Oct 2020 11:57:54 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     Dmitry Osipenko <digetx@...il.com>, joro@...tes.org,
        vdumpa@...dia.com, jonathanh@...dia.com,
        linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device()

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.

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.

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.

Thierry

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