drm/i915/execlists: Check preempt-timeout target before submit_ports

We evaluate *active, which is a pointer into execlists->inflight[]
during dequeue to decide how long a preempt-timeout we need to apply.
However, as soon as we do the submit_ports, the HW may send its ACK
interrupt causing us to promote execlists->pending[] tp
execlists->inflight[], overwriting the value of *active. We know *active
is only stable until we submit (as we only submit when there is no
pending promotion).

[   16.102328] BUG: KCSAN: data-race in execlists_dequeue+0x1449/0x1600 [i915]
[   16.102356]
[   16.102375] race at unknown origin, with read to 0xffff8881e9500488 of 8 bytes by task 429 on cpu 1:
[   16.102780]  execlists_dequeue+0x1449/0x1600 [i915]
[   16.103160]  __execlists_submission_tasklet+0x48/0x60 [i915]
[   16.103540]  execlists_submit_request+0x38e/0x3c0 [i915]
[   16.103940]  submit_notify+0x8f/0xc0 [i915]
[   16.104308]  __i915_sw_fence_complete+0x61/0x420 [i915]
[   16.104683]  i915_sw_fence_complete+0x58/0x80 [i915]
[   16.105054]  i915_sw_fence_commit+0x16/0x20 [i915]
[   16.105457]  __i915_request_queue+0x60/0x70 [i915]
[   16.105843]  i915_gem_do_execbuffer+0x2d6b/0x4230 [i915]
[   16.106227]  i915_gem_execbuffer2_ioctl+0x2b0/0x580 [i915]
[   16.106257]  drm_ioctl_kernel+0xe9/0x130
[   16.106279]  drm_ioctl+0x27d/0x45e
[   16.106311]  ksys_ioctl+0x89/0xb0
[   16.106336]  __x64_sys_ioctl+0x42/0x60
[   16.106370]  do_syscall_64+0x6e/0x2c0
[   16.106397]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
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/20200426094231.21995-1-chris@chris-wilson.co.uk

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

@@ -2438,8 +2438,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		clear_ports(port + 1, last_port - port);
 
 		WRITE_ONCE(execlists->yield, -1);
-		execlists_submit_ports(engine);
 		set_preempt_timeout(engine, *active);
+		execlists_submit_ports(engine);
 	} else {
 skip_submit:
 		ring_set_paused(engine, 0);