drm/i915/execlists: Mark up the racy access to switch_priority_hint

[ 7534.150687] BUG: KCSAN: data-race in __execlists_submission_tasklet [i915] / process_csb [i915]
[ 7534.150706]
[ 7534.150717] write to 0xffff8881f1bc24b4 of 4 bytes by task 24404 on cpu 3:
[ 7534.150925]  __execlists_submission_tasklet+0x1158/0x2780 [i915]
[ 7534.151133]  execlists_submit_request+0x2e8/0x2f0 [i915]
[ 7534.151348]  submit_notify+0x8f/0xc0 [i915]
[ 7534.151549]  __i915_sw_fence_complete+0x5d/0x3e0 [i915]
[ 7534.151753]  i915_sw_fence_complete+0x58/0x80 [i915]
[ 7534.151963]  i915_sw_fence_commit+0x16/0x20 [i915]
[ 7534.152179]  __i915_request_queue+0x60/0x70 [i915]
[ 7534.152388]  i915_gem_do_execbuffer+0x3997/0x4c20 [i915]
[ 7534.152598]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[ 7534.152615]  drm_ioctl_kernel+0xe4/0x120
[ 7534.152629]  drm_ioctl+0x297/0x4c7
[ 7534.152642]  ksys_ioctl+0x89/0xb0
[ 7534.152654]  __x64_sys_ioctl+0x42/0x60
[ 7534.152667]  do_syscall_64+0x6e/0x2c0
[ 7534.152681]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 7534.152693]
[ 7534.152703] read to 0xffff8881f1bc24b4 of 4 bytes by interrupt on cpu 2:
[ 7534.152914]  process_csb+0xe7c/0x10a0 [i915]
[ 7534.153120]  execlists_submission_tasklet+0x30/0x170 [i915]
[ 7534.153138]  tasklet_action_common.isra.0+0x42/0xa0
[ 7534.153153]  __do_softirq+0xd7/0x2cd
[ 7534.153166]  run_ksoftirqd+0x15/0x20
[ 7534.153180]  smpboot_thread_fn+0x1ab/0x300
[ 7534.153194]  kthread+0x19a/0x1e0
[ 7534.153207]  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/20200309144249.10309-1-chris@chris-wilson.co.uk

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

@@ -1782,12 +1782,13 @@ timeslice(const struct intel_engine_cs *engine)
 static unsigned long
 active_timeslice(const struct intel_engine_cs *engine)
 {
-	const struct i915_request *rq = *engine->execlists.active;
+	const struct intel_engine_execlists *execlists = &engine->execlists;
+	const struct i915_request *rq = *execlists->active;
 
 	if (!rq || i915_request_completed(rq))
 		return 0;
 
-	if (engine->execlists.switch_priority_hint < effective_prio(rq))
+	if (READ_ONCE(execlists->switch_priority_hint) < effective_prio(rq))
 		return 0;
 
 	return timeslice(engine);
@@ -1804,8 +1805,11 @@ static void set_timeslice(struct intel_engine_cs *engine)
 static void start_timeslice(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists *execlists = &engine->execlists;
+	int prio = queue_prio(execlists);
 
-	execlists->switch_priority_hint = execlists->queue_priority_hint;
+	WRITE_ONCE(execlists->switch_priority_hint, prio);
+	if (prio == INT_MIN)
+		return;
 
 	if (timer_pending(&execlists->timer))
 		return;
@@ -5330,10 +5334,10 @@ void intel_execlists_show_requests(struct intel_engine_cs *engine,
 
 	if (execlists->switch_priority_hint != INT_MIN)
 		drm_printf(m, "\t\tSwitch priority hint: %d\n",
-			   execlists->switch_priority_hint);
+			   READ_ONCE(execlists->switch_priority_hint));
 	if (execlists->queue_priority_hint != INT_MIN)
 		drm_printf(m, "\t\tQueue priority hint: %d\n",
-			   execlists->queue_priority_hint);
+			   READ_ONCE(execlists->queue_priority_hint));
 
 	last = NULL;
 	count = 0;