On 07/10/2014 10:21 AM, Chris Wilson wrote:
Jerome Glisse pointed out that get_user_pages() does not synchronize with concurrent invalidations of the VMA. As such if the backing vma is changed whilst the pages for the object are being grabbed for use by the GPU, we may end up with a random mixture of page references being held. Worse still as the mmu-notifier will believe that the VMA invalidation was complete and the old page references dropped. In order to serialise gup with mmu-notifier, we use a seqlock to detect when an invalidation has occurred in parallel to our gup and if so cancel the gup. The detection is a little coarse, but hopefully we never see contention here!
Looks good to me, but as we discussed on IRC all get_user_pages users have this "problem" and I don't understand why it matters to us? How would someone hit/exploit the race? By one thread manically modifying mappings while another is creating userptr objects? Is there some other legitimate way of it happening?
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