drm/i915: don't clobber userspace memory before commiting to the pread
The pagemap.h prefault helpers do the prefaulting by simply writing some data into every page. Hence we should not prefault when we're not yet commited to to actually writing data to userspace. The problem is now that - we can't prefault while holding dev->struct_mutex for we could deadlock with our own pagefault handler - we need to grab dev->struct_mutex before copying to sync up with any outsanding gpu writes. Therefore only prefault when we're dropping the lock the first time in the pread slowpath - at that point we're committed to the write, don't wait on the gpu anymore and hence won't return early (with e.g. -EINTR). Tested-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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