linux-kernel - Re: [PATCH 07/20] iommu/amd: Remove special mapping code for dma

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Date:	Tue, 12 Jul 2016 13:48:06 +0200
From:	Joerg Roedel <jroedel@...e.de>
To:	Robin Murphy <robin.murphy@....com>
Cc:	Joerg Roedel <joro@...tes.org>, iommu@...ts.linux-foundation.org,
	Vincent.Wan@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/20] iommu/amd: Remove special mapping code for dma_ops
 path

Hi Robin,

On Tue, Jul 12, 2016 at 12:42:59PM +0100, Robin Murphy wrote:
> Ah, that's the angle I was missing, yes. So if, say, someone is mapping
> a page at IOVA 0x0000 while someone else is mapping a page at 0x1000,
> there could still be a race between both callers writing the non-leaf
> PTEs, but it's benign since they'd be writing identical entries anyway.
> Seems reasonable to me (I assume in a similar map vs. unmap race, the
> unmapper would just be removing the leaf entry, rather than bothering to
> check for empty tables and tear down intermediate levels, so the same
> still applies).

The non-leaf PTE setup code checks for races with cmpxchg, so we are on
the safe side there. Two threads would write different entries there,
because they are allocating differnt sub-pages, but as I said, this is
checked for using cmpxchg. On the PTE level this problem does not exist.


	Joerg