[Intel-gfx] [PATCH 1/2] drm/i915: Don't drop pinned fences

[Chris Wilson]

On Mon, Jan 27, 2014 at 09:49:47PM +0100, Daniel Vetter wrote:
> Userspace can currently provoke this when e.g. trying to use a pinned
> scanout as a cursor or overlay target. Later on that might lead to
> some fun fence pin count mayhem.
> 
> Spurred by Ville's report that something goes wrong here and
> originally I've thought that this might slip through the pwrite gtt
> fastpath. But that one checks of obj tiling, so should be ok.
> 
> But one thing that _does_ blow up is the vma unbinding with more than
> one address space. The next patch will fix this.

It's buggy behaviour, so if (WARN_ON(fence->pin_count) return -EBUSY;

I did worry that we would then have some unexpected failure, a new EBUSY
returning to userspace, but I think along the paths that can trigger it,
we have already returned EBUSY in the (dim and distant) past.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre