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

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Message-ID: <0c0d3707-9ea5-44f9-88a1-a65c62e3df8d@arm.com>
Date: Wed, 21 Jan 2026 13:03:42 +0000
From: Robin Murphy <robin.murphy@....com>
To: Danilo Krummrich <dakr@...nel.org>, Wang Jiayue <akaieurus@...il.com>,
 hanguidong02@...il.com, gregkh@...uxfoundation.org, rafael@...nel.org
Cc: 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, joro@...tes.org,
 iommu@...ts.linux.dev
Subject: Re: [PATCH v5] driver core: enforce device_lock for
 driver_match_device()

On 2026-01-21 11:02 am, 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);
> 
>   #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);
> 
> @qcom maintainers: I'm aware of commit 0b4eeee2876f ("iommu/arm-smmu-qcom:
> Register the TBU driver in qcom_smmu_impl_init"), but I think the above patch
> should work fine as it is still *not only* registered when
> CONFIG_ARM_SMMU_QCOM_DEBUG?

In principle there should be nothing wrong with registering the driver 
unconditionally - that existing tbu_registered logic looks racy in the 
face of async_probe anyway - however I don't think the *_platform_driver 
macros will work here, as this all gets combined into arm_smmu.ko 
wherein ending up with multiple module_init declarations breaks the build.

(Please do double-check all the build permutations of ARM_SMMU, 
ARM_SMMU_QCOM and ARM_SMMU_QCOM_DEBUG)

Thanks,
Robin.