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

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Message-ID: <20241209142013.GC1888283@ziepe.ca>
Date: Mon, 9 Dec 2024 10:20:13 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Cc: 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: Re: [PATCH v2] iommu/tegra241-cmdqv: do not use smp_processor_id in
 preemptible context

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@...hat.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@...dia.com>

Jason