drm/i915/gt: Make intel_ring_unpin() safe for concurrent pint

In order to avoid some nasty mutex inversions, commit 09c5ab3
("drm/i915: Keep rings pinned while the context is active") allowed the
intel_ring unpinning to be run concurrently with the next context
pinning it. Thus each step in intel_ring_unpin() needed to be atomic and
ordered in a nice onion with intel_ring_pin() so that the lifetimes
overlapped and were always safe.

Sadly, a few steps in intel_ring_unpin() were overlooked, such as
closing the read/write pointers of the ring and discarding the
intel_ring.vaddr, as these steps were not serialised with
intel_ring_pin() and so could leave the ring in disarray.

Fixes: 09c5ab3 ("drm/i915: Keep rings pinned while the context is active")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118230254.2615942-6-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_ring.c

@@ -57,9 +57,10 @@ int intel_ring_pin(struct intel_ring *ring)
 
 	i915_vma_make_unshrinkable(vma);
 
-	GEM_BUG_ON(ring->vaddr);
-	ring->vaddr = addr;
+	/* Discard any unused bytes beyond that submitted to hw. */
+	intel_ring_reset(ring, ring->emit);
 
+	ring->vaddr = addr;
 	return 0;
 
 err_ring:
@@ -85,20 +86,14 @@ void intel_ring_unpin(struct intel_ring *ring)
 	if (!atomic_dec_and_test(&ring->pin_count))
 		return;
 
-	/* Discard any unused bytes beyond that submitted to hw. */
-	intel_ring_reset(ring, ring->emit);
-
 	i915_vma_unset_ggtt_write(vma);
 	if (i915_vma_is_map_and_fenceable(vma))
 		i915_vma_unpin_iomap(vma);
 	else
 		i915_gem_object_unpin_map(vma->obj);
 
-	GEM_BUG_ON(!ring->vaddr);
-	ring->vaddr = NULL;
-
-	i915_vma_unpin(vma);
 	i915_vma_make_purgeable(vma);
+	i915_vma_unpin(vma);
 }
 
 static struct i915_vma *create_ring_vma(struct i915_ggtt *ggtt, int size)