[PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor_id in preemptible context

[PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor_id in preemptible context

[Luis Claudio R. Goncalves]

During boot some of the calls to tegra241_cmdqv_get_cmdq() will happen
in preemptible context. As this function calls smp_processor_id(), if
CONFIG_DEBUG_PREEMPT is enabled, these calls will trigger a series of
"BUG: using smp_processor_id() in preemptible" backtraces.

As tegra241_cmdqv_get_cmdq() only calls smp_processor_id() to use the
CPU number as a factor to balance out traffic on cmdq usage, it is safe
to use raw_smp_processor_id() here.

v2: Sebastian helped identify that the problem was not exclusive to kernels
    with PREEMPT_RT enabled. The delta between v1 and v2 is the description.

Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
---
 drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c b/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c
index fcd13d301fff6..3ea4369e838fe 100644
--- a/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c
+++ b/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c
@@ -339,7 +339,7 @@ tegra241_cmdqv_get_cmdq(struct arm_smmu_device *smmu,
 	 * one CPU at a time can enter the process, while the others
 	 * will be spinning at the same lock.
 	 */
-	lidx = smp_processor_id() % cmdqv->num_lvcmdqs_per_vintf;
+	lidx = raw_smp_processor_id() % cmdqv->num_lvcmdqs_per_vintf;
 	vcmdq = vintf->lvcmdqs[lidx];
 	if (!vcmdq || !READ_ONCE(vcmdq->enabled))
 		return NULL;
-- 
2.47.0

Re: [PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor_id in preemptible context

[Jason Gunthorpe]

On Fri, Dec 06, 2024 at 10:01:14AM -0300, Luis Claudio R. Goncalves wrote:
> During boot some of the calls to tegra241_cmdqv_get_cmdq() will happen
> in preemptible context. As this function calls smp_processor_id(), if
> CONFIG_DEBUG_PREEMPT is enabled, these calls will trigger a series of
> "BUG: using smp_processor_id() in preemptible" backtraces.
> 
> As tegra241_cmdqv_get_cmdq() only calls smp_processor_id() to use the
> CPU number as a factor to balance out traffic on cmdq usage, it is safe
> to use raw_smp_processor_id() here.
> 
> v2: Sebastian helped identify that the problem was not exclusive to kernels
>     with PREEMPT_RT enabled. The delta between v1 and v2 is the description.
> 
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Makes sense

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason

Re: [PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor_id in preemptible context

[Nicolin Chen]

On Fri, Dec 06, 2024 at 10:01:14AM -0300, Luis Claudio R. Goncalves wrote:
> During boot some of the calls to tegra241_cmdqv_get_cmdq() will happen
> in preemptible context. As this function calls smp_processor_id(), if
> CONFIG_DEBUG_PREEMPT is enabled, these calls will trigger a series of
> "BUG: using smp_processor_id() in preemptible" backtraces.

Confirmed the BUG prints. Should we CC stable tree?

> As tegra241_cmdqv_get_cmdq() only calls smp_processor_id() to use the
> CPU number as a factor to balance out traffic on cmdq usage, it is safe
> to use raw_smp_processor_id() here.
> 
> v2: Sebastian helped identify that the problem was not exclusive to kernels
>     with PREEMPT_RT enabled. The delta between v1 and v2 is the description.
> 
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>

Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>

Thanks
Nicolin

Re: [PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor_id in preemptible context

[Will Deacon]

On Fri, 06 Dec 2024 10:01:14 -0300, Luis Claudio R. Goncalves wrote:
> During boot some of the calls to tegra241_cmdqv_get_cmdq() will happen
> in preemptible context. As this function calls smp_processor_id(), if
> CONFIG_DEBUG_PREEMPT is enabled, these calls will trigger a series of
> "BUG: using smp_processor_id() in preemptible" backtraces.
> 
> As tegra241_cmdqv_get_cmdq() only calls smp_processor_id() to use the
> CPU number as a factor to balance out traffic on cmdq usage, it is safe
> to use raw_smp_processor_id() here.
> 
> [...]

Applied to will (for-joerg/arm-smmu/fixes), thanks!

[1/1] iommu/tegra241-cmdqv: do not use smp_processor_id in preemptible context
      https://git.kernel.org/will/c/1f806218164d

Cheers,
-- 
Will

https://fixes.arm64.dev
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