drm/i915/execlists: Mark up data-races in virtual engines

The virtual engine passes tokens back and forth to its backing physical
engines.

[   57.372993] BUG: KCSAN: data-race in execlists_dequeue [i915] / virtual_submission_tasklet [i915]
[   57.373012]
[   57.373023] write to 0xffff8881f47324c0 of 4 bytes by interrupt on cpu 2:
[   57.373241]  execlists_dequeue+0x6fa/0x2150 [i915]
[   57.373458]  __execlists_submission_tasklet+0x48/0x60 [i915]
[   57.373677]  execlists_submission_tasklet+0xd3/0x170 [i915]
[   57.373694]  tasklet_action_common.isra.0+0x42/0xa0
[   57.373709]  __do_softirq+0xd7/0x2cd
[   57.373723]  irq_exit+0xbe/0xe0
[   57.373735]  do_IRQ+0x51/0x100
[   57.373748]  ret_from_intr+0x0/0x1c
[   57.373963]  engine_retire+0x89/0xe0 [i915]
[   57.373977]  process_one_work+0x3b1/0x690
[   57.373990]  worker_thread+0x80/0x670
[   57.374004]  kthread+0x19a/0x1e0
[   57.374017]  ret_from_fork+0x1f/0x30
[   57.374027]
[   57.374038] read to 0xffff8881f47324c0 of 4 bytes by interrupt on cpu 3:
[   57.374256]  virtual_submission_tasklet+0x27/0x5a0 [i915]
[   57.374273]  tasklet_action_common.isra.0+0x42/0xa0
[   57.374288]  __do_softirq+0xd7/0x2cd
[   57.374302]  run_ksoftirqd+0x15/0x20
[   57.374315]  smpboot_thread_fn+0x1ab/0x300
[   57.374329]  kthread+0x19a/0x1e0
[   57.374342]  ret_from_fork+0x1f/0x30
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200310141320.24149-3-chris@chris-wilson.co.uk

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

@@ -1004,7 +1004,7 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
 				i915_request_cancel_breadcrumb(rq);
 				spin_unlock(&rq->lock);
 			}
-			rq->engine = owner;
+			WRITE_ONCE(rq->engine, owner);
 			owner->submit_request(rq);
 			active = NULL;
 		}
@@ -2033,13 +2033,14 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 				     "",
 				     yesno(engine != ve->siblings[0]));
 
-			ve->request = NULL;
-			ve->base.execlists.queue_priority_hint = INT_MIN;
+			WRITE_ONCE(ve->request, NULL);
+			WRITE_ONCE(ve->base.execlists.queue_priority_hint,
+				   INT_MIN);
 			rb_erase_cached(rb, &execlists->virtual);
 			RB_CLEAR_NODE(rb);
 
 			GEM_BUG_ON(!(rq->execution_mask & engine->mask));
-			rq->engine = engine;
+			WRITE_ONCE(rq->engine, engine);
 
 			if (engine != ve->siblings[0]) {
 				u32 *regs = ve->context.lrc_reg_state;
@@ -4944,7 +4945,7 @@ static intel_engine_mask_t virtual_submission_mask(struct virtual_engine *ve)
 static void virtual_submission_tasklet(unsigned long data)
 {
 	struct virtual_engine * const ve = (struct virtual_engine *)data;
-	const int prio = ve->base.execlists.queue_priority_hint;
+	const int prio = READ_ONCE(ve->base.execlists.queue_priority_hint);
 	intel_engine_mask_t mask;
 	unsigned int n;