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Date:   Tue, 20 Aug 2019 02:41:43 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Tom Murphy <murphyt7@tcd.ie>
Cc:     iommu@lists.linux-foundation.org, Heiko Stuebner <heiko@sntech.de>,
        virtualization@lists.linux-foundation.org,
        linux-tegra@vger.kernel.org,
        Thierry Reding <thierry.reding@gmail.com>,
        Will Deacon <will@kernel.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        linux-samsung-soc@vger.kernel.org,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        linux-rockchip@lists.infradead.org, Andy Gross <agross@kernel.org>,
        Gerald Schaefer <gerald.schaefer@de.ibm.com>,
        linux-s390@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        linux-mediatek@lists.infradead.org,
        Matthias Brugger <matthias.bgg@gmail.com>,
        linux-arm-kernel@lists.infradead.org,
        David Woodhouse <dwmw2@infradead.org>,
        linux-kernel@vger.kernel.org, Kukjin Kim <kgene@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH V5 1/5] iommu/amd: Remove unnecessary locking from AMD
 iommu driver
Message-ID: <20190820094143.GA24154@infradead.org>
References: <20190815110944.3579-1-murphyt7@tcd.ie>
 <20190815110944.3579-2-murphyt7@tcd.ie>
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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.