linux-kernel - Re: [PATCH V5 1/5] iommu/amd: Remove unnecessary locking from AMD iommu driver

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Message-ID: <20190820094143.GA24154@infradead.org>
Date:   Tue, 20 Aug 2019 02:41:43 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Tom Murphy <murphyt7@....ie>
Cc:     iommu@...ts.linux-foundation.org, Heiko Stuebner <heiko@...ech.de>,
        virtualization@...ts.linux-foundation.org,
        linux-tegra@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Will Deacon <will@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        linux-samsung-soc@...r.kernel.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-rockchip@...ts.infradead.org, Andy Gross <agross@...nel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        linux-s390@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        David Woodhouse <dwmw2@...radead.org>,
        linux-kernel@...r.kernel.org, Kukjin Kim <kgene@...nel.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH V5 1/5] iommu/amd: Remove unnecessary locking from AMD
 iommu driver

On Thu, Aug 15, 2019 at 12:09:39PM +0100, Tom Murphy wrote:
> We can remove the mutex lock from amd_iommu_map and amd_iommu_unmap.
> iommu_map doesn’t lock while mapping and so no two calls should touch
> the same iova range. The AMD driver already handles the page table page
> allocations without locks so we can safely remove the locks.

I've been looking over the code and trying to understand how the
synchronization works.  I gues we the cmpxchg64 in free_clear_pte
is the important point here?  I have to admit I don't fully understand
the concurrency issues here, but neither do I understand what the
mutex you removed might have helped to start with.