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

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Message-Id: <DFU7SVI5SYE5.3MJP2ECGGZK5P@kernel.org>
Date: Wed, 21 Jan 2026 12:02:15 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "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>, <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 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?