[Intel-gfx] [PATCH] drm/i915: Reorder context-close to avoid calling i915_vma_close() under RCU

[Chris Wilson]

Quoting Chris Wilson (2017-11-09 08:20:48)
> When we close the VMA, we unbind it from the ppgtt and tear down the
> page directory pointing at it. That may trigger us to return WC pages
> back to the system, requiring conversion back to WB which itself may
> sleep. That makes i915_vma_close() unsuitable for use inside the RCU
> read lock, which we need to hold to iterate the radixtree.
> 
> The fix is quite simple, we can close all the VMA as we close the ppgtt,
> we only need to do that instead of closing them during destruction of
> the LUT.

The flaw is that i915_vma_close() frees the vma; destroying our
backpointer from the lut to the object. Definitely time for tea. :(
-Chris