drm/i915/selftests: Disable preemption while setting up fence-timers

The impossible happens and a future fence expired while we were still
initialising. The probable cause is that the test was preempted and we
lost our scheduler cpu slice. Disable preemption during this test to
rule out preemption as a source of timer disruption.

References: https://bugs.freedesktop.org/show_bug.cgi?id=110039Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190313205944.5768-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_sw_fence.c

@@ -571,21 +571,27 @@ static int test_timer(void *arg)
 	unsigned long target, delay;
 	struct timed_fence tf;
 
+	preempt_disable();
 	timed_fence_init(&tf, target = jiffies);
 	if (!i915_sw_fence_done(&tf.fence)) {
 		pr_err("Fence with immediate expiration not signaled\n");
 		goto err;
 	}
+	preempt_enable();
 	timed_fence_fini(&tf);
 
 	for_each_prime_number(delay, i915_selftest.timeout_jiffies/2) {
+		preempt_disable();
 		timed_fence_init(&tf, target = jiffies + delay);
 		if (i915_sw_fence_done(&tf.fence)) {
 			pr_err("Fence with future expiration (%lu jiffies) already signaled\n", delay);
 			goto err;
 		}
+		preempt_enable();
 
 		i915_sw_fence_wait(&tf.fence);
+
+		preempt_disable();
 		if (!i915_sw_fence_done(&tf.fence)) {
 			pr_err("Fence not signaled after wait\n");
 			goto err;
@@ -595,13 +601,14 @@ static int test_timer(void *arg)
 			       target, jiffies);
 			goto err;
 		}
-
+		preempt_enable();
 		timed_fence_fini(&tf);
 	}
 
 	return 0;
 
 err:
+	preempt_enable();
 	timed_fence_fini(&tf);
 	return -EINVAL;
 }