[Intel-gfx] [PATCH] drm/i915: Mark up racy read of rq->engine

[Chris Wilson]

As the i915_request.engine may be updated by a virtual engine to either
point to the virtual engine or the real physical engine on submission,
we have to be wary that the engine pointer may change.

[  213.317076] BUG: KCSAN: data-race in execlists_dequeue [i915] / i915_request_wait [i915]
[  213.317097]
[  213.317110] write (marked) to 0xffff8881e8647650 of 8 bytes by interrupt on cpu 2:
[  213.317386]  execlists_dequeue+0x43b/0x1670 [i915]
[  213.317645]  __execlists_submission_tasklet+0x48/0x60 [i915]
[  213.317905]  execlists_submission_tasklet+0xd3/0x170 [i915]
[  213.317926]  tasklet_action_common.isra.0+0x42/0x90
[  213.317943]  __do_softirq+0xc8/0x206
[  213.317958]  irq_exit+0xcd/0xe0
[  213.317980]  irq_work_interrupt+0xf/0x20
[  213.317999]  __tsan_read8+0x30/0x100
[  213.318272]  retire_requests+0xdd/0xf0 [i915]
[  213.318502]  engine_retire+0xa6/0xe0 [i915]
[  213.318519]  process_one_work+0x3af/0x640
[  213.318534]  worker_thread+0x80/0x670
[  213.318548]  kthread+0x19a/0x1e0
[  213.318566]  ret_from_fork+0x1f/0x30
[  213.318584]
[  213.318595] read to 0xffff8881e8647650 of 8 bytes by task 458 on cpu 1:
[  213.318847]  i915_request_wait+0x3e3/0x510 [i915]
[  213.319088]  i915_gem_object_wait_fence+0x81/0xa0 [i915]
[  213.319328]  i915_gem_object_wait+0x26b/0x560 [i915]
[  213.319578]  i915_gem_wait_ioctl+0x141/0x290 [i915]
[  213.319597]  drm_ioctl_kernel+0xe9/0x130
[  213.319613]  drm_ioctl+0x27d/0x45e
[  213.319628]  ksys_ioctl+0x89/0xb0
[  213.319648]  __x64_sys_ioctl+0x42/0x60
[  213.319666]  do_syscall_64+0x6e/0x2c0
[  213.319680]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

In this case, we are merely trying to flush the most recent engine
associated with the request, and do not care which as the process of
chaing the engine is done by a tasklet, with which we are yielding to.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/i915_request.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 22635bbabf06..e9fd20242438 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1660,7 +1660,7 @@ long i915_request_wait(struct i915_request *rq,
 			break;
 		}
 
-		intel_engine_flush_submission(rq->engine);
+		intel_engine_flush_submission(READ_ONCE(rq->engine));
 
 		if (signal_pending_state(state, current)) {
 			timeout = -ERESTARTSYS;
-- 
2.20.1