linux-kernel - Re: [PATCH v4 2/2] iommu/arm-smmu-v3: Improve CMDQ lock fairness and efficiency

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Message-ID: <aSXlAT00HVCGBaNY@willie-the-truck>
Date: Tue, 25 Nov 2025 17:18:57 +0000
From: Will Deacon <will@...nel.org>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>
Cc: linux-kernel@...r.kernel.org,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Joerg Roedel <joro@...tes.org>, Mostafa Saleh <smostafa@...gle.com>,
	Jason Gunthorpe <jgg@...dia.com>,
	Robin Murphy <robin.murphy@....com>,
	Nicolin Chen <nicolinc@...dia.com>,
	Zhang Yu <zhangyu1@...ux.microsoft.com>,
	Jean Philippe-Brucker <jean-philippe@...aro.org>,
	Alexander Grest <Alexander.Grest@...rosoft.com>
Subject: Re: [PATCH v4 2/2] iommu/arm-smmu-v3: Improve CMDQ lock fairness and
 efficiency

On Fri, Nov 14, 2025 at 09:17:18AM -0800, Jacob Pan wrote:
> @@ -521,9 +527,14 @@ static bool arm_smmu_cmdq_shared_tryunlock(struct arm_smmu_cmdq *cmdq)
>  	__ret;								\
>  })
>  
> +/*
> + * Only clear the sign bit when releasing the exclusive lock this will
> + * allow any shared_lock() waiters to proceed without the possibility
> + * of entering the exclusive lock in a tight loop.
> + */
>  #define arm_smmu_cmdq_exclusive_unlock_irqrestore(cmdq, flags)		\
>  ({									\
> -	atomic_set_release(&cmdq->lock, 0);				\
> +	atomic_fetch_and_release(~INT_MIN, &cmdq->lock);		\

nit: you can use atomic_fetch_andnot_release(INT_MIN)

That aside, doesn't this introduce a new fairness issue in that a steady
stream of shared lockers will starve somebody trying to take the lock
in exclusive state?

Will