drm/i915: Handle recursive shrinker for vma->last_active allocation

If we call into the shrinker for direct relcaim inside kmalloc, it will
retire the requests. If we retire the vma->last_active while processing a
new i915_vma_move_to_active() we can upset the delicate bookkeeping
required for the cache. After the possible invocation of the shrinker, we
need to double check the vma->last_active is still valid.

Fixes: 8b293eb53a7d ("drm/i915: Track the last-active inside the i915_vma")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105600#c39
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180719072206.16015-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_vma.c

@@ -942,6 +942,14 @@ static struct i915_gem_active *active_instance(struct i915_vma *vma, u64 idx)
 	}
 
 	active = kmalloc(sizeof(*active), GFP_KERNEL);
+
+	/* kmalloc may retire the vma->last_active request (thanks shrinker)! */
+	if (unlikely(!i915_gem_active_raw(&vma->last_active,
+					  &vma->vm->i915->drm.struct_mutex))) {
+		kfree(active);
+		goto out;
+	}
+
 	if (unlikely(!active))
 		return ERR_PTR(-ENOMEM);