linux-kernel - [PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor

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Message-ID: <Z1L1mja3nXzsJ0Pk@uudg.org>
Date: Fri, 6 Dec 2024 10:01:14 -0300
From: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
To: Thierry Reding <thierry.reding@...il.com>,
	Krishna Reddy <vdumpa@...dia.com>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Jonathan Hunter <jonathanh@...dia.com>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-devel@...ts.linux.dev
Subject: [PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor_id in
 preemptible context

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@...hat.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