linux-kernel - Re: [PATCH v5] driver core: enforce device_lock for driver_match

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Message-ID: <2026012126-coke-lash-dd99@gregkh>
Date: Wed, 21 Jan 2026 12:19:10 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Wang Jiayue <akaieurus@...il.com>, hanguidong02@...il.com,
	rafael@...nel.org, Aishwarya.TCV@....com, broonie@...nel.org,
	chenqiuji666@...il.com, linux-kernel@...r.kernel.org,
	m.szyprowski@...sung.com, robin.clark@....qualcomm.com,
	will@...nel.org, robin.murphy@....com, joro@...tes.org,
	iommu@...ts.linux.dev
Subject: Re: [PATCH v5] driver core: enforce device_lock for
 driver_match_device()

On Wed, Jan 21, 2026 at 12:02:15PM +0100, Danilo Krummrich wrote:
> On Wed Jan 21, 2026 at 11:40 AM CET, Danilo Krummrich wrote:
> > So, the problem is that in the callstack of the arm-smmu driver's (a platform
> > driver) probe() function, the QCOM specific code (through arm_smmu_impl_init())
> > registers another platform driver. Since we are still in probe() of arm-smmu the
> > call to platform_driver_register() happens with the device lock of the arm-smmu
> > platform device held.
> >
> > platform_driver_register() eventually results in driver_attach() which iterates
> > over all the devices of a bus. Since the device we are probing and the driver we
> > are registering are for the same bus (i.e. the platform bus) it can now happen
> > that by chance that we also match the exact same device that is currently probed
> > again. And since we take the device lock for matching now, we actually take the
> > same lock twice.
> >
> > Now, we could avoid this by not matching bound devices, but we check this
> > through dev->driver while holding the device lock, so that doesn't help.
> >
> > But on the other hand, I don't see any reason why a driver would call
> > platform_driver_register() from probe() in the first place. I think drivers
> > should not do that and instead just register the driver through a normal
> > initcall.
> >
> > (If, however, it turns out that registering drivers from probe() is something we
> > really need for some reason, it is probably best to drop the patch and don't
> > make any guarantees about whether match() is called with the device lock held or
> > not.
> >
> > Consequently, driver_override must be protected with a separate lock (which
> > would be the cleaner solution in any case).)
> 
> I assume that this should resolve the problem (unless there are more drivers
> that register drivers in probe()):
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> index 573085349df3..9bb793efc35f 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> @@ -774,10 +774,6 @@ struct arm_smmu_device *qcom_smmu_impl_init(struct arm_smmu_device *smmu)
>  {
>         const struct device_node *np = smmu->dev->of_node;
>         const struct of_device_id *match;
> -       static u8 tbu_registered;
> -
> -       if (!tbu_registered++)
> -               platform_driver_register(&qcom_smmu_tbu_driver);

Ick, yeah, that should not be happening.  We should deadlock on that no
matter what.

> 
>  #ifdef CONFIG_ACPI
>         if (np == NULL) {
> @@ -802,3 +798,5 @@ struct arm_smmu_device *qcom_smmu_impl_init(struct arm_smmu_device *smmu)
> 
>         return smmu;
>  }
> +
> +builtin_platform_driver(qcom_smmu_tbu_driver);

change makes sense to me.

thanks,

greg k-h