drm/i915/selftests: Drop struct_mutex around lowlevel pggtt allocation

For a ppgtt that we are constructing, there is no struct_mutex
dependence so skip it. In the process, also ping the scheduler
frequently to try and avoid the NMI watchdog.

v2: gen6 requires struct_mutex to clean up (currently)

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=107094
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/20180703135331.12265-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -156,12 +156,9 @@ static int igt_ppgtt_alloc(void *arg)
 	if (!USES_PPGTT(dev_priv))
 		return 0;
 
-	mutex_lock(&dev_priv->drm.struct_mutex);
 	ppgtt = __hw_ppgtt_create(dev_priv);
-	if (IS_ERR(ppgtt)) {
-		err = PTR_ERR(ppgtt);
-		goto err_unlock;
-	}
+	if (IS_ERR(ppgtt))
+		return PTR_ERR(ppgtt);
 
 	if (!ppgtt->vm.allocate_va_range)
 		goto err_ppgtt_cleanup;
@@ -180,6 +177,8 @@ static int igt_ppgtt_alloc(void *arg)
 			goto err_ppgtt_cleanup;
 		}
 
+		cond_resched();
+
 		ppgtt->vm.clear_range(&ppgtt->vm, 0, size);
 	}
 
@@ -197,13 +196,15 @@ static int igt_ppgtt_alloc(void *arg)
 			}
 			goto err_ppgtt_cleanup;
 		}
+
+		cond_resched();
 	}
 
 err_ppgtt_cleanup:
+	mutex_lock(&dev_priv->drm.struct_mutex);
 	ppgtt->vm.cleanup(&ppgtt->vm);
-	kfree(ppgtt);
-err_unlock:
 	mutex_unlock(&dev_priv->drm.struct_mutex);
+	kfree(ppgtt);
 	return err;
 }