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

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Message-ID: <aTjk89lJFc3sNTN_@google.com>
Date: Wed, 10 Dec 2025 12:11:47 +0900
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 Sun, Nov 30, 2025 at 02:52:36PM -0800, Jacob Pan wrote:
> On Tue, 25 Nov 2025 17:18:57 +0000
> Will Deacon <will@...nel.org> wrote:
> > 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?
> > 
> I don't think this change will starve exclusive lockers in the
> current code flow since new shared locker must acquire exclusive locker
> first while polling for available queue spaces.

Looking at this again, we already have the same starvation problem in
that the lockword has to hit zero for the exclusive locker to succeed.

So my initial worry was unfounded.

Will